OK well the problem recurred earlier when the wife fired up the PC so I did some fiddling. Moving the case and leads didn't make any difference or restore the image so I connected another monitor (an old 4:3 one) using the same lead and on restarting the PC manually (couldn't shut down normally as no screen was visible) the thing seemed to boot up ok and the Windows icon formed on the screen but then the screen suddenly went blue and a 'no signal message' appeared. I waited a while but nothing was happening so I turned the PC off/on again and the same thing happened again. I tried starting in safe mode and everything loaded up and I was able to access her desktop and open pictures etc. I then turned off again and reconnected the original monitor and bingo, this time the process completed and I was able to use the PC as normal. I again moved the case and leads around a bit but this didn't seem to affect anything and the PC's been on and working OK since then.
She did mention something which also happened this morning at some point when she was trying to get the thing working she got a black/white screen appear which apparently looked like the safe mode restart options screen but said something about error repair (or words similar). She didn't select that option but just tried to restart the PC again manually so I have no idea if that screen was related to this issue or something else entirely. It didn't appear again when I was fiddling around.
