DEAR LUPIN by Roger Mortimer and Charlie Mortimer TWO-hander plays based on letters between famous people have become a bit of a cliche over the last few decades.

They are easy and cheap to stage. All you need are two well-known actors, using their natural charm to recreate the wit and wisdom of other well-known people who wrote to each other a lot.

But Dear Lupin – based on the letters written by celebrated racing correspondent Roger Mortimer to his wayward son Charlie – is a treat.

Charlie published his late dad’s witty and teasing letters, bringing Roger a posthumous second fame, and now the book has become a play starring James Fox and his son Jack Fox.

What gives this adaptation by Michael Simkins real edge is Charlie’s emergence as a character in his own right – in the book he stayed firmly in the background.

It falls to Jack Fox to hold the play together, as Charlie becomes our personal 'guide’ to his own louche life and his dad’s quirky reaction to it. Drink, drugs and full indulgence in the 1970s sexual revolution all play their part.

Charlie is a man capable of extraordinary contrariness – to the point of undergoing a harrowing army training, just to walk away when his acceptance as an officer is assured.

Mr Fox junior delivers a performance of dizzying energy, at one point leaping over the furniture on the set to illustrate his army training. He is constantly on the move – maddening and charming, the living embodiment of a generation that had no idea what it wanted.

It is a delicious irony that his dad – the great James Fox – has publicly admitted to living a louche 1960s life himself. Here he is perfect as Charlie’s dad Roger. Their relationship will ring a bell with anyone who had a long-suffering, tolerant father they only truly appreciated when it was too late.

Mr Fox senior is an old hand at playing likeable, tolerant English gentlemen. He is in his element as Roger, gently doling out deliciously dry advice to his son or telling riotous meandering tales. But the play lets him step briefly out of character several times to become other figures in his son’s life.

He becomes an ageing prostitute, an army sergeant major and a foulmouthed dodgy antique dealer among other things.

Prepare to be moved as father and son decline together in health – one through age, one though illness. These scenes, under the delicate control of director Philip Franks, are heart-stopping.

We should all be grateful Charlie survived in the end to immortalise his dad’s letters and make this play possible.

l Dear Lupin runs until tomorrow (Saturday) at Theatre Royal Windsor, in Thames Street. Evening performances 8pm. Saturday matinee 3pm. To book visit www.theatreroyalwindsor.co.uk or call the box office on 01753 853 888.

Coming soon: Avenue Q. Monday April 27-Saturday May 2. Evening performances 8pm. Matinees Thursday 2.30pm, Saturday 3pm.