Hey, it's that voice!
I happened to be listening to the 1976 BBC Radio adaptation of Strong Poison today. It has a few familiar names in the cast list: Gabriel Woolf as Inspector Parker, Peter Jones as Bunter, Stephen Thorne as the Attorney-General, and one Christopher Bidmead as Ryland Vaughan. The one that caught me unawares, though, was in episode 5. Miss Climpson, an elderly spinster, is making a solo investigation, and has to gain the confidence of an unsuspecting nurse. And the mark is played by none other than ( You've guessed it )