''Godfrey's Ghost'' is, at its heart, the story of a father written by his son. As a young man, Arnold Ridley was chiefly known as the author of the long-running comedy thriller, ''The Ghost Train''. Today he is remembered for his television performances as Private Godfrey, the oldest member of the Dad's Army platoon. But Army came towards the close of a long life, and although Arnold Ridley was Private Godfrey, Private Godfrey was not Arnold Ridley. From humble beginnings living above the family boot-shop in Bath, through the horrors of the Great War, to the life of a celebrated playwright in London's West End. From unprecedented success and sudden wealth to unforeseen disaster and financial ruin. From the nightmare of another war through painful years of struggle and obscurity to a new, late-flowering fame in a television classic. Anecdotal, engaging and truthful, ''Godfrey's Ghost'' is an affectionate - and often moving - portrait of a remarkable man who, faced with more than his fair share of life's vicissitudes, through love, courage and the kind of well-grounded philosophy that doesn't recognise itself as such, lived his life valiantly and well.