Agatha Christie’s Witness for the Prosecution, which I review today, is a play I first saw when I acted in it. Let me explain more clearly: I knew nothing of its script, or its outcome, until I made a first-night appearance in a professional touring production of it on the New Theatre stage, on July 26, 1979.

I had been asked by a colleague to play Mr Barker, junior counsel for the prosecution at the murder trial of Leonard Voles. The idea, of course, was that I would write about it. Barker was a non-speaking role; all I had to do was shuffle papers and hold what I hoped looked like learned whispered exchanges with other barristers. Oh, and of course follow the action with interest.

The fact that I had never seen the play before meant that I paid more than usually close attention to the trial. As I wrote in the Oxford Mail the next day: “Last night’s audience . . . sat pondering the all-important question of ‘whodunnit?’. They might have been intrigued to learn that someone on stage was every bit as baffled.”

So who did do it? Watching the play again at Milton Keynes last week, I hadn’t a clue. But isn’t that always the way with the Queen of Crime? Wasn’t it Ogden Nash who said, in a poem I can’t now lay my hands on, that a Christie made the perfect desert island book, because no matter how many times you read it, you can never remember who commits the murder?