UPDATE:
Somewhat of a delay in reporting this as I ended up in hospital with appendicitis last week...
Anyway, processor turned up. Only PC spares place near me was Maplins, so off I go there to get some heatsink paste. Bought some (the paste cost more than I paid for the processor
), came home and pulled PC apart.
CPU did indeed just 'pop' out when the heatsink was removed and the arm was released. New one dropped into place, a bit of paste on top and then the heatsink clipped straight on.
Connected everything up, fired up computer and all went well. Ran the CPU identifying software that someone recommended and now I do indeed have a P3 700mhz 'Coppermine' processor instead of a piece of Celery!
Best of all, my MAME emulator runs properly again and I can play all my favourite retro arcade games
Including the ones that I couldn't play on my old PC because it was too slow
The computer also seems to be more stable, Firefox and Explorer run better and slightly faster and in general the whole system seems to be marginally faster.
So a P3 700mhz CPU is definitely a lot better than a Celeron 600mhz. :slayer:
Many thanks to Stuart Demon, Trans and Liam for all your help and advice. Cheers guys! :slayer: