Rob,
First thing (if not done before) is to clean the computer case, heaters and fans (without forgetting the power supply) with compressed air (either in can, or with a compressor for spray painting, or at a car repair shop) [my English may not be accurate... , but I guess you will understand]
Nota: to be done outdoors or you will see an angry wife giving you the vacuum cleaner...
2nd nota: this have to be done very year for the sake of the computer parts... (I know few people are doing it, but if you do your will have less failure and a computer OK for several years).
If you want to change the CPU heater and fan, you can look at e-Bay for Akasa, Arctic Cooling or Zallman (not an exhaustive list of good brands).
I saw 2 Akasa models in UK there (look for "akasa athlon").
PS: may be some wool from the sheeps clogging the heater...
