It's mostly Sky-run channels (e.g. Sci-Fi, which I believe is run out of someone's back bedroom... ;) ) so it's likely the broadcaster. (They also skimp on things like making sure the programme's finished before running the continuity announcements, squeezing up the screen, et cetera...) I think they own everything right up to the box, so I probably shouldn't go messing with it (especially given my technical ineptitude), but when I get square with them again I'll ask about a replacement box just in case.

Many thanks for further enlightenment!