Oh, he's one of those been-in-everything character actors - Doctor Who three times, guested in nearly everything right into the last decade, was in the original Forsyte Saga and played Nicholas Nickleby for the BBC back in the 60s, plus he and his wife also part-time live in LA, so he's guested in all the usual US suspects as well as the Brit ones.

The reason I wrote the post title like that is because it is a standing joke that he is in everything on BBC Radio! He's always done a lot of it and does have a fab voice, but his audio work is huge. I've found at least three spoof Martin Jarvis BBC radio credits just going through the Genome listing. (eg. "Martin Jarvis reads the Haynes manual" XD)