The room temp was normal (21'C) but the PC may have been cold last couple times I've tried - it's winter and it gets cold in here!
Playing with it last night and it seemed to be getting worse... It had been on for a couple hours and I still get maybe 1 in 4 key presses doubled up. I've tried scrubbing the motherboard around where the keyboard connector is with contact cleaner but if anything it's worse for a little while after doing that! I can't see any physical damage or corrosion or anything around that corner.
I don't have another PS/2 keyboard to hand to test (have one on order), but this one tests fine in another PC (an old ThinkPad) so I don't think it's the keyboard.
Thing is that worries me is that I did a BIOS update soon after receiving this PC and it didn't seem to have this problem back before I did it. It might be a coincidence, or it might not. I can't downgrade back to the version it was on before - it doesn't seem to support going back a major release. I'm sure the BIOS is stable though as it was a very common office PC back in the day. I was on v1.o5 when I got it and now I'm on latest 2.02.I can go back to 2.01 easily enough but the issue is still there.
Today I turned it on this morning and it did the same thing, but left it alone for an hour and it's absolutely fine now. Maybe I'll have to take a closer look at these surface mount capacitors all over the place...
Cheers,
Steve