UP & RUNNING AGAIN
Well, spend a couple of hours with the machine in bits. Reseated the RAM sticks, graphics card, firewire card in fact just about everything that could be disconnected and refitted. Finally gave it a good clean out with the vac (do they supply the dust and fluff when you buy the machines?) and put it all back together again. Fired it up and all is good. Don't know which fix actually worked but let's see if it continues to run. Thanks for the advice guys, much appreciated.