I don't actually know that my late great-uncle worked at Bletchley. But the available evidence suggests he did. Here's what I do know.

I inherited a few things when he died - not in his will, but just collected from his attic. One of these is significant, and linked.

There was a card table (I used it for a computer keyboard); several decks of cards (useless, all missing a single card); a set of bridge score pads (bridge is an incredible game); and a heavy metal box. Really heavy: we moved it from Manchester to Scotland when the family relocated, it took two of us to lift. (Not two removal men, obviously. those guys can lift washing machines.) The box sat in my bedroom when I was a teenager; I put a lamp on top it. The lock is unlike anything I've seen elsewhere: one catch with two letter dials, one catch with two numbers, and they both have to be right to open.

I think my uncle must have BUILT the box himself (he was an engineer.) He didn't leave the code written down anywhere. Well, I thought he didn't. He did, in fact, but I didn't find it for several years. I did SEE it, however. And I saw it -- at Bletchley Park.

The reason I was at Bletchley was unusual. I was there working on a movie - The Imitation Game, which stars Benedict Cumberbatch as Turing. He had, apparently, requested a mathematician to talk to, to help him play the character better -- and they wanted some mathematics for a notebook.

They hired me. (I was very cheap, and knew someone on the production staff.) There's a shot of a notebook in the movie -- I can't find a still of it now, but it's there, and it's in my handwriting (my very best handwriting, which is almost legible). Benedict, incidentally, is really nice. I also met Kiera Knightley, but she was understandably polite but reserved around strange mathematicians.

Anyway, that was the first time I'd been to Bletchey, ever. There's quite a fancy museum there now, but then it was just mouldering huts. The Bombe (or a replica? I'm actually not sure) was in a leaky building. It was hard to tell what was original and what had been built for the film, anyway.

In between takes and lunch, I went exploring, and found a hut full of photographs. Like I say, I don't know if it was material for the film or archival. There were no signs - also no barriers. One photograph caught my eye: it was a group of men and women around a complex-looking machine.

I think I noticed it because I was thinking about movies, and movie-making, and it reminded me of the monolith in 2001. So I gave it a closer look. In the corner of the photograph - almost cut-off by the frame - was a mark in pen. I didn't think anything of it at the time, of course.

But now, looking back, I can see it clearly. I wish I'd taken a photo! But I'm not even sure I had a camera phone back then. And maybe I'm remembering it wrong. But I don't think I am, because when I saw that code again in a bridge pad, I recognised it. TR49.

- Jon, Narrative Director