Well, I've got a lot further!
I've identified the vertical deflection circuit and checked all the electrolytic capacitors - two had failed and have now been replaced. I found a replacement uPC1031 IC and replaced that (it appears to use an IC rather than distinct transistors). I've tested the vertical part of the yoke, and the resistance is the same as a known working machine (that sadly, I don't have to hand, otherwise I'd swap tubes). There's no obvious fusible resistors to replace.
I'm a little stuck what to do next - any suggestions? I still get a horizontal line, and if twiddle v-lin (I think) a bit, I can get the line to be about 5mm thick - at that point I can make out a bit of text, so I know the computer is fine, and presumably the horizontal circuits are too. I'm stuck!
Many thanks in advance.