So. On Tuesday afternoon I went out and bought a new printer. On Wednesday DH spent over an hour getting it installed (including having to ring their helpline).
We got most of the cards printed. DH is doing some work on the house so needed me to switch off while he checked out fuses. And when I tried to turn the computer back on it wouldn't boot up.
After trying several times, finding the recovery disk and trying that - no luck, we gave up and went to do something about supper. At which point we found that the cooker wouldn't turn on. DH investigated the other fuse box and couldn't get the trip switch to stay on. Eventually DH realised that the work on the first fuse box had caused an earth problem. Sorted that out. Decided that maybe we should check the computer again. Still no luck.
Start fidling round with things and I suddenly wondered what happened if we unplugged the printer.
BINGO. Unplug the printer and the computer boots up fine. Try again with the printer turned on - also fine.
So it turns out that somewhere in the boot up the system checks the USB port and if the printer is plugged in but not turned on the system hangs.