It really was one of the best days ever. I remember Richard sat on the sofa in Cameron's office and absolutely cried tears of joy that all of their original stuff was intact, but also relief that we'd caught and complemented their spirit in our songs.

It's hard to imagine being given a project that is more brilliant or rewarding than Mary Poppins. It's also true to say that it was extremely terrifying at first.

We were given carte blanche by Cameron to pretty much do what we wanted with the existing Sherman brothers' songs as well as to write or develop the new ones it was felt were needed for the story on stage which is not the story of the film and owes a lot to the much darker original books by PL Travers.

I mean, how far do you go and mess around with classic songs that we have both known since we were six years old? That was the terrifying side of the thrill.

We'd been working on the songs for over a year and a half by the time we got to Bristol, so there wasn't a bar of the show we hadn't arranged - and there are over three thousand of them.

You'd hear people coming out saying that 'Temper Temper' was definitely in the film, and yet it's one of ours, and everyone came out humming 'Supercalifragilistic...' even though its 80 per cent new lyrics"

In the film there aren't many 'ocious' rhymes so I had to make them up - the Shermans used 'atrocious' and 'precocious' but not 'ferocious', so I put that in, then made up my own like 'Check your breath before you speak in case it's halitoicious'.

My favourite is: 'Add some further flourishes, it's so rococococious'! "

I was in the kitchen making coffee with Cameron and George was in the lounge tinkling on the piano and Cameron heard him and said 'Oh I like the sound of that. What is it?' I said I thought it was the new sequence when the children misbehave - which it was.

But George hadn't actually written anything.

He just busked - and rhythmically it sounded a bit angry, like "temper, temper".

So almost on the spot we came up with an idea for a song. Of course, Cameron loved it.

It is always strange to be invited on to a project that is already in your life. When I was cast as the Minister for Defense in Tomorrow Never Dies, it was quite a long time before I got over the oddness of saying lines such as, 'You have three days, Mr Bond,' when I had spent so many hours, from the age of nine onwards, watching Mr Bond taking similar orders from men in suits. Now, I was giving them.

In the same way, when the impresario Cameron Mackintosh telephoned me on the set of Monarch of the Glen and asked if I would adapt Mary Poppins for the stage, the very name of the mythical nanny triggered all sorts of childhood memories: being read to in the bath by my aunt and begging her for more hot water so I could stay there until the end of the chapter; seeing the reflected door as I stared into the night-time window of our nursery in Wetherby Place; climbing out among the chimneypots, high above Hereford Square.

The latter, needless to say, would have given my parents - well, my mother anyway heart failure. Nowadays, there would probably be a stern admonition from the publishers: 'Do not try this at home.' But I did.

When Pamela Travers's creation first appeared in the 1930s the world of cooks and nannies and odd-job men was as ordinary and reassuring as rain or Christmas.

But the second big promotion of her books came in the 1950s, my era, when the austerity of rationing, the privations of war and the general dreariness engulfing Britain at that time lent a new burst of nostalgia to the safe and ordered world of Mary Poppins' charges, Jane and Michael Banks. Of course, as a little boy, I was unaware of which aspects of English life had gone missing since 1939 but there was something that immediately spoke to me in the stories.

They were all about the Secret Life of Children and the key to Mary's fascination was; in the delicious contrast between her exterior as a severe and traditional nanny, convincing to adults in every way, and the magical adventures that she could spirit you off. to at any moment. It is this duality that Julie Andrews embodies so convincingly in the Disney film of 1964.

The subtlety with which she introduces a sense of the wittier aspects of Travers' heroine makes Andrews, in Disney terms, almost subversive. It is no wonder that Travers, an extremely difficult woman as even her friends would concede, never had anything but praise for the actress. Andrews's only deficiency being, apparently, that she was too pretty and that was 'really not her fault'.

The call from Cameron came in October 2002 when I was still warm from the glow of my Oscar for writing Gosford Park, and the lucky recipient of several beguiling approaches, but I knew at once that Mary Poppins would be different. This wasn't just a tempting job offer. It was a chance to work on something iconic, a part of the British and American collective consciousness.

Not many authors create a genuine childhood myth, Lewis Carroll and JM Barrie, of course, but few others could claim membership of this select company - besides, that is, the complicated figure of Pamela Travers.

But as I started to work, I soon realised that the chief challenge ahead lay in the Poppins generation gap. When I listened to the stories as I played with my rubber ducks it was Travers's creation, illustrated by Mary Shepard, who lodged in my mind. For my wife Emma, however, 15 years younger than me, Mary Poppins is Julie Andrews and vice versa. How was the show to be un disappointing to fans of the books as well as those of the much loved film? It would have to be the child of both.

To facilitate things, before the project came my way, a historic treaty had been forged. In a nutshell, Disney had never purchased the stage rights and these had set off on a different journey, finally, to Travers's delight, finding shelter with Cameron Mackintosh.

However, Cameron, a showman to his fingertips, knew that he needed the Disney songs for his musical. This impasse might have lasted indefinitely but for the intervention of the infinitely subtle Tom Schumacher, the veteran producer of such stage hits as The Lion King and The Hunchback of Notre Dame.

At a stroke, the Gordian knot was cut and work could commence on the first ever Disney co-production. So, we had the songs we wanted but the film's settings for them might not always work on stage. My first job, therefore, was to mine the books for alternatives. Disney had flirted with some of these in the puzzling opening scene when the (mercifully) inimitable Dick Van Dyke, as Bert the chimneysweep, sings to a group of characters who are never seen again: Mrs Corry and her daughters, Miss Lark, Miss Persimmon, and so on. Maybe this was to pave the way for a sequel that never happened.

Whatever the reason for it, the magical Mrs Corry and her bullied offspring soon found their way from the stories into the show. The books tell us that Jane and Michael Banks are impossible, going through nannies by the dozen, but, with all due respect, there is little evidence of this in the film, which seemed a pity. Before-long, the chapters 'Bad Tuesday' and 'Bad Wednesday' had become the 'Temper, Temper' sequence, which is one of our strongest. And so it went on.

The Poppins story must revolve around the redemption of the children's distant father, George Banks.

Travers's father died young and the yearning for her lost parent infuses everything she writes. Her mother, on the other hand, interested her little and, as a result, Mrs Banks makes no impact in the books. It is therefore necessary for any adaptor essentially to celebrate the one parent and to invent the other. Disney created the 'run on the bank' for George and suffragettism for Winifred. I was happy with a bank plot for him, which I felt could be slightly more real, but she presented a problem. 'Votes for Women' is no longer a comedic slogan and trying to revisit it would be patronising and tasteless.

What I arrived at in the end was a woman who is striving to be her husband's perfect wife and her children's perfect mother, but who has lost herself in the process. Add a husband with no sense of proportion who is the victim of a harsh upbringing, and we have a dysfunctional family that needs rescuing. As part of this, we revived Travers's Miss Andrew, the wicked nanny who blighted George's life - the enemy, in other words, of everything Mary stands for.

The film's wonderful songs had, of course, been written by the Sherman brothers and their support for the show has been unstinting, but the new (brilliant) numbers, as well as some reinvention of the originals, were undertaken by the composer George Stiles and the lyricist Anthony Drewe. Accordingly, when Cameron felt it was time to bring together the two writing elements - words and music - we three were summoned to his home and presented to each other.

Knowing that someone will be in your presence for the next two years and in your life for the rest of it puts a certain spin on meetings such as these, and I can now confess to being taken aback by the two trendy young men, dressed for lunch in Santa Monica, waiting for me. They, on the other hand, hoping for a vibrant, Oscar-winning, up to-the-minute screenwriter, were confronted by an old fart in corduroys and tweeds, who looked more like a refugee from the Countryside March than a West End collaborator. The following week we met in their basement studio near Baron's Court in west London, whither I stumbled, trembling. We never looked back.

I am happy to put on public record that George and Ants pulled me through the entire experience.

The veterans of several musicals, including the award-winning Honk and The Just So Stories, they dried my tears and puffed me up by turns through the months ahead. I could never adequately express. My gratitude. Naturally, we had our battles but I cannot remember one important struggle where we were not all three on the same side.

Then came the director. Richard Eyre is perhaps best known for his work as director of the National Theatre and in the world of the classics; nevertheless his production of Guys and Dolls had been one of the highlights of musical theatre in recent years. For me, his appearance could not have been better timed. Travers's books are a rich feast of stories and incidents, almost all of which had their supporters among the team, and the script was in danger of drowning in an embarras de richesses. Richard cut through the fatty tissue in the early drafts to expose the meat.

Matthew Bourne, the brilliant creator of the all-male Swan Lake as well as the recent Play Without Words and The Nutcracker, came on board as choreographer and co-director. Bob Crowley, serial Tony award winner, was appointed designer. A lustrous group indeed, always presided over by Cameron as King Edward VII with Tom Schumacher playing Lord Esher, eminence grise, behind the scenes.

There were high points and low points, both during the Poppins weekends at Cameron's beautiful house in Somerset, where together we would play through the developing versions. In these sessions, Cameron always insisted on the part of Michael Banks until, regretfully, he had at last to relinquish it.

My own specialty was Mrs Brill the housekeeper, closely based, I admit, on Angela Baddeley's Mrs Bridges in Upstairs, Downstairs. I did once attempt Mary. At the end of that particular reading, Cameron announced that now we all knew what Mary Poppins would be like if she were played by Mrs. Thatcher, but there was no need to repeat the experiment.

Our only real hurdle along the way was the rehearsed reading for Michael Eisner, the CEO of Disney, whose support was a sine qua non. This was a nervous-making time even though, as it happens, both he and his wife, Jane, were delightful. Hearts thumping, we climbed to the attics above the Old Vie Theatre and, after some coffee and murmured chatter, we began. The script had not reached its final form but the musical numbers were there and the readers gave it everything they had. The result was a nod of approval and, from then on, it was full steam ahead.

There is a rhythm in inventing a musical and there comes a time when the writer, initially so central to its creation, drops back as others arrive to make his work their own. By the spring of this year it was time for the readings, the Somerset weekends, the days in the studio and the nights at my desk to give way and for the show to begin. There are also stages to be observed in these things.

First comes the script for the start of work. Next is the script we will open with. A month of playing brings changes. And so on. This creates a hiatus between the completion of the rehearsal script and the work commencing, so when we all assembled at the Saddler's Wells Theatre on July 19, I entered the room., to an extent, as a born-again virgin, ready to be surprised by everything. I had played no part in the casting and I did not recognize some of the faces who would embody the characters I had been slaving over, although there were also, of course, old favourites.

I was especially pleased by David Haig as George but my revelation came in Linzi Hateley, a newcomer to me, as his wife. This Winifred Banks owes little to her forerunner in either film or book, she is an original creation, and it was thrilling to watch Linzi breathe warmth into her, making her moments among the most touching in the show.

Our Bert, Gavin Lee, is a star in the making and, as Mary, Laura Michelle Kelly is one-of those people whom a benign God allows to get through without a flaw, charming and lovely and with a voice whose soaring, effortless beauty makes you cry To quote a thank-you note from a school friend of my son Peregrine's: 'On the whole, I was satisfied.'

By the time I returned to rehearsals, things were moving on.

Matthew Bourne and Stephen Mear, his marvelous co-choreographer, had created the dazzling dance routines, songs which I had heard sung only by George and Ants were blended into the performances, and the piece was coming alive. There is an excitement every time you hear lines, imagined in the bath or while stuck in a traffic jam on the A3 I, being spoken as thoughts and feelings, and so often being rendered deeper or funnier by the actors than they were when they left your brain. This is the reward of writing which never - so far anyway - palls.

A summons to Bristol, the cradle of the production, for publicity, brought more evidence of Poppins Fever. When you work on a project, you can be so involved in the immediate problems of narrative or staging that it is easy to forget the wider context in which it will be received. I remember being asked whether I had felt an Oscar was in the offing when we were making Gosford Park. Truly, you have no sense of anything beyond the demands of today's scenes. Do the altered lines work? Have we lost any information? Can we introduce a new element at this point in the story? Now, here I was, with members of the press and television, talking about the 'world premiere' of this project that had only been played, for the previous two years, in a studio at Baron's Court or in the London and country palaces of Cameron Mackintosh.

As a trailer of coming delights, Bristol had shown me Bob Crowley's amazing recreation of Number I7 Cherry Tree Lane, with its computer grids, flying rooms and elevators from roof to stage, and once again I was struck by the strange and surprising journey between the moment when you write, 'Bert is on the roof, looking out over the night sky,' and the seismic operation necessary to make those few words flesh. After that, it hardly seemed a moment before I was sitting in the stalls, with Emma by my side, as the curtain rose on the cluster of chinmeypots.

In the interests of good taste, I must resist excessive gush and chatter but it had, after all, been an elephantine pregnancy since I took that long-ago telephone call in a Nissen hut in Inverness-shire, and I think I may be forgiven for saying that, as the audience rose to its feet in a torrent of cheering applause, I felt, and indulged, the pride of a parent when the child has been safely delivered.