Hello GWN,
Thanks for all your help with this problem. Next time I get a BSOD, I'll start a new topic on bleepingcomputer. Strangely, I haven't had one for some time, but I've been using the PC almost every day recently and, as I say, they generally seem to happen when the PC has been dormant for some time.
Ultimately, I need a new computer, but just want to keep this one going a few months longer. It's not that I'm particularly attached to Vista (it's probably the worst version of Windows I've used, and I go back to 3.0!) it's more that I need a new car more than I need a new computer at the moment (it's on its last legs) so want to stick with the PC a bit longer. It might also give the chance for these phenomenally expensive 4k computers to come down in price a bit.
I've tried running the WSUS Offline Update tool, just to see what happened. It claimed to have installed a few things (kb3191203, for example), but I can't see any evidence for it. The files the patch is supposed to update haven't been updated. It then started checking for updates, but I killed it off after about 4 hours. I guess that's just a manifestation of the general Vista / Win 7 slow update problem. At some time over the next few weeks, I'll try running WSUS overnight to see if that gets me anywhere.
Once again, thanks for your help.