Hi guys - I posted a while back saying I had found a guy who sorted out this problem for me - his name is Lee and works in South Woodford, London, right next to the tube station in a small computer repair shop .
He replaced the chip, and the thermal paste. In all honesty the solution lasted about months, and then the same thing happened. I took it back, and he was quite surprised, and so agreed that I only pay for the chip and not the labour this time round. So he charged me £60 as opposed to the £180 he charged last time. Im pretty sure he changed the chip as he gave me the old one!
If it happens again then I guess it will be time to get a new mac! I will try and get his website details if anyone needs it.