First, I'd like to thank everyone that has read and commented. Very welcome.

Just a further update:

Had the 7600 graphics card back today. Well, actually it's a NEW 7600 graphics card, so something must have been up there. Good service there from PC Nextday - eleven months after buying it............

Further thought: Fiddling about the other day, I took the heatsink off the CPU and cleaned it. Not that dusty, but need to be thorough. What I did notice was that the thermal grease between it and the CPU looked degraded. Looked more like toothpaste on the bathroom mirror. So I've ordered a tube (I've read all the articles about not putting it on like butter - a 'rice bead' is what I understand is recommended).

Just another piece to the puzzle. One day, I might get back to having a PC that ran the way it used to!