Well, the new processor that arrived this morning did the trick .. even though I haven't taken it out of the box yet
Tried taking all but one memory module out - it started up fine
Tried pairing up a second - easy peasy
Put a third in - everything working
Put the fourth in expecting that to stop it from POSTing - but the thing carried on anyway.
I'm confused - if the memory works fine, not sure what the problem was before. But I'm not knocking it
Going to reinstall XP now (new mobo, new graphics card) and when that's all working switch CPUs
Apologies for all the smilies
I'm just relieved I have a working PC at last