ONE of TV's best-loved comedies first saw the light of day on a train journey from Victoria to Stratford East. Actor Jimmy Perry was on his way across London to work with Joan Littlewood's Theatre Workshop company and took the opportunity to jot down notes for an idea for a comedy series that had come to him a few days earlier.

He still has that ancient school notebook - one with weights, measures and multiplication tables on the back cover - where Dad's Army first saw the light of day. It's a remarkable series that's as popular today as it ever was in the 1960s and 1970s when it ran for 80 episodes and spawned film, radio and stage versions.

Characters such as Captain Mainwaring, the pompous bank manager who led the motley crew of soldiers, and Private "stupid boy" Pike remain as funny today despite the rise (and fall) of alternative comedy and a whole new battalion of funsters.

Perry was writing from experience. As a 16-year-old, he'd served in the original Dad's Army, established to protect the country from invasion during the Second World War. So in 1967 when he had the idea for the series he decided to brush up his facts. Imagine his surprise when he went to Westminster public library and was told "never heard of it" by the girl behind the desk. A trip to the Imperial War Museum yielded little more information. The Home Guard had been all but forgotten - a situation that Perry and his co-writer David Croft did more than a little to remedy. Now The Complete A-Z Of Dad's Army, a history of the series and the people who made it, has been written by Richard Webber with the cooperation of Perry and Croft.

The result is everything you ever wanted to know (and possibly a few things you didn't want to know) about the series with cast lists, previously unpublished photos, character and actor profiles, and a whole lot more. It reveals, for instance, that the pilot for an American version called The Rear Guard was recorded in 1976 but didn't convince executives to press ahead with a series.

Before finding fame as Sir Humphrey in Yes Minister, actor Nigel Hawthorne was seen as "the angry man" in an episode. The reason for his fury was Lance Corporal Jones borrowing his bike. He's not the only now-famous face to join the army game. Carry On and EastEnders star Barbara Windsor appeared in the sixth episode as stage performer Laura La Plaz. Wendy Richard, who's been Pauline Fowler in EastEnders for the past 15 years, played two characters in the TV series and two in the radio version. John Bardon, who now appears as Jim Branning in EastEnders, played Private Walker in the Dad's Army stage show. Jack Haig played Lance Corporal Jones for part of the theatre run before going on to play Monsieur Leclerc in five series of 'Allo 'Allo.

Arnold Ridley, who played Private Godfrey, actually served in the army in the First World War, being invalided out in 1917 after being wounded at the Somme. As well as acting work, he wrote The Ghost Train, the classic comedy thriller that's still being performed on stage nearly 80 years after being written.

Bud Flanagan who sang the theme song, Who Do You Think You Are Kidding Mr Hitler, was part of the Crazy Gang with Chesney Allen. The fictitious town of Walmington-on-Sea is situated on the south coast of England but nearly all location filming took place in Norfolk, in and around the town of Thetford.

Tony Pritchard, of the Dad's Army Appreciation Society, has made it a personal crusade to find the whereabouts of all the locations used. Tony and his wife have used family holidays in the area in the past three years to scour the region for sites where scenes were shot.

A treatment for a second Dad's Army film was written and Laurence Olivier, a great fan of the show, indicated he was interested in playing the leading role of the villain. But Perry and Croft dropped the idea because they were busy and didn't think the first film succeeded.

Jimmy Perry went on to use his own experiences as the basis for other TV comedies - serving as a sergeant in the Far East (It Ain't Half Hot Mum), as a Butlins redcoat (Hi-De-Hi!) and his grandfather's stories of being a gentleman's gentleman (You Rang, M'Lord?).

Even now, over three decades since its debut, the sitcom pulls in big audiences. The first series attracted modest audiences of eight million but was pulling in 12 million by the second series, reaching a peak of over 16 million viewers an episode during the fifth season. The Dad's Army Appreciation Society, formed in 1993, has over 1,000 members. Its commander-in-chief is retired civil servant Jack Wheeler who watches at least three episodes of the sitcom every evening.

Members range from eight to 80 and span the globe. A museum dedicated to the memory of Dad's Army opened at Bressingham Steam Museum, Norfolk, in May. Sets based on the original designs show visitors what life would have been like in Walmington. It looks as if Dad's Army will be marching on for years to come.

l The Complete A-Z Of Dad's Army is published by Orion, £20.