Thanks @Zippy Zapp
Well, I started attempting to check the traces, but a combination of lack of time and concern that I might end up scraping too much off the contacts off led me to leave it for the time being.
However, yesterday I received the new keyboard membrane which works a treat... apart from...
It has the same issues with the c, v, n & m keys. Only this time, there was a slightly different issue. The left mouse was occasionally not working. More precisely, it wasn't working for selecting icons, although pressing on the left alt (to select the icon) a few times, it also worked.
Because both of these issues are sort of consistent, and at this point I don't really feel that it's software (otherwise it would be more consistent), I'm beginning to suspect that it's linked to the power supply.
So, I've now checked the pins with a multimeter, and they don't appear to be correct:
The top left pin (+12v) was fluctuating close to +1v
The middle pin (-12v) was fluctuating around -2.2v
The bottom-right pin (+5v) was pretty steady at 3.3v
I believe (please correct me if I'm wrong) that the 12v rails are for CD / floppy / HDD, and something related to audio, so I feel I'm lucky that the compact flash card requires very little draw from that.
I'm also thinking that because the 5v rail is lacking power, then that could explain the strange behaviour of the keyboard and mouse (mouse issue seems more inconsistent - it's a USB mouse plugged via an adapter).
Does this sound plausible?
Can anyone recommend a good power supply? Either something that's relatively simple to fit in to the original power supply case (not the wedge-shaped one), or a recommended pre-built alternative?