It's one of the most famous names in showbiz, and not just because of its owner's massive success as an actor: Benedict Cumberbatch is one of those appellations that seems to enjoy tripping off the tongue. Benedict Cumberbatch. Benedict. Cumberbatch. BENEDICTCUMBERBATCH. Ahem.

It's been corrupted every which way, from his fans the "Cumberbitches" to the old joke about him starring in a TV drama about a man working on an allotment, during which the director shouts: "Cue Cumberbatch!" One famous article in the normally august Washington Post even repeatedly referred to him as Bandersnatch Cummerbund, for a laugh. Other nicknames for the actor are, sadly, unprintable.

All this being the case, you'd think Cumberbatch himself would be intimately familiar with the origins of his surname. So when RadioTimes.com met him recently (yes, that happened – get over it), we checked – and found he doesn't have any idea.

"I don't know," Cumberbatch said. "I genuinely don't. Nobody does. There are theories: an 11-year-old girl I was acting with in Creation, a film I did with Paul Bettany, came up to me and said, 'I know what your name means!' I said, great, tell me! I've been trying to find out all my life. She said, 'It means small valley dweller.' I said, I'm going to pretend you didn't tell me that."

Someone who dwells in a small valley, or a small person who dwells in a valley?

"I think a very big person in a small valley! But I immediately got images of dank stone cottages. Kind of romantic, but a bit depressing."

There is indeed a Welsh village called Cwmbach: "small valley". So that could be the answer. Any other possibilities?

"The other genealogy of it could be that it came over with the Norman conquest, but that's like saying everyone's related to the Queen Mum. [I think it's] probably [from] Germany."

Well, this just wasn't good enough. We wanted to know and we suspected you would too, so we thought: who do we know who'll know? Easy: Susie Dent, resident of Countdown's dictionary corner and, indeed, the puzzles page in Radio Times magazine.

After a bit of digging, Susie came back to us and said she reckons that 11-year-old girl was almost right, but not quite spot-on: "In a book of place names that existed before the Norman Conquest, 'comberbach' is listed as a stream in the Weaver valley of north Cheshire. The two parts of 'Cumberbatch' are 'cumbra', originally a byword for a Cumbrian and later an Anglo-Saxon personal name popular in North West England, and 'bach', meaning a 'beck', or stream in a valley.

"Together they make 'Cumbra's beck'. So 'Cumberbatch' is a surname based on a very specific place - Benedict's ancestors will have lived in that very valley in north Cheshire that is still there today."

Put the kettle on, Mrs Hudson - we've cracked the case! Or have we? Do you know better? Is your hero of German stock? If you've got your own theory or research, post it in the comments below...