THIS is an evening defined by the well-turned phrases of a man who made his living honing and finely crafting words.

But it is also about much more. For while the bon mots of men such as Doctor Johnson and Oscar Wilde forever sit coldly on the page wallowing in their cleverness, these letters from a highly literate father to his errant son are filled with an unrepentant honesty.

They reverberate with the greatest emotion of all… the sometimes long-suffering but never less than unconditional love that a parent feels for a child.

There is nothing remotely conversational about this prose. No one actually talks like this - the words gleam precisely because of the elbow grease applied by someone who fully understands the tricks of their trade.

Roger Mortimer was a former soldier, prisoner-of-war and journalist. Educated at Eton, he must have had high hopes for his son. But then the 1960s and the new hedonism arrives.

Mortimer senior must stand by and watch as his beloved boy descends into a world of sex, drink, drugs and rock ‘n’ roll. He even blows his chances of a commission in the British Army’s premier regiment.

Yet the father never gives up... there is nothing this prodigal son can do that might dim the flame of a parent’s faith that everything might turn out all right in the end.

Real-life father and son James and Jack Fox are Roger and Charlie Mortimer, the latter having the pet name Lupin of the title. Their performances are variously comical, tinged with sadness and disappointment, but never less than outstandingly true to reality.

It is a moving study of human frailties, faults, foibles but above all the endless capacity for forgiveness that only people who share the same genes can feel for each other.

Adapted for the stage by Michael Simkins, this magnificent production will strike a chord with all those for whom the distant echoes of parental expectation still resonate.

Dear Lupin runs until Saturday (May 16).

John Phillpott