Thanks for the replies. Seems it may just be case of confirming what I already suspect :(
Ok, heatsink is not dusty. I am (and have been running for the life of the card) an Accelero S1 (the big version) with a 120mm fan on the top.
At rest temps are approx 48C and at load the max I can...
I'm running an E7200 processor, P5Q-Pro, 2 x 2 GB Geil Ram with a Radeon HD 4850 all powered by a Corsair 520W PSU. I use windows 7 home premium 64 bit. Everything has been running nicely for almost 2 years (upgraded from win XP to win 7 when it came out) and I've been able to play most games...