Once, when he was on a train, Denis heard a couple of men talking about wanting a cigarette but they didn't have any. He did, so he offered them some. He didn't want any payment, but one of them said that he must accept something, so did this sketch of him.
To me, that doesn't look like the kind of sketch an average person on a train would have been able to do, and the fact that it's in colour (it's actually much more subtly-coloured than my scan makes out) suggests someone in the habit of carrying coloured pencils around in their pocket. Add to this that it's an uncanny likeness, and it makes me wonder if the artist was actually someone quite famous.
I don't know if it's signed, and I'm sure Denis's children aren't ever going to part with it, but it would be intriguing to find out who it was who could knock this off on a train in return for a cigarette.