Thanks for your help guys. it's idling at 24/25 with OC now, so it looks like its ok.
One strange thing has happened though. during startup, i get some beeps during post, and a message appears saying "problem with CPU fan. press F3 to resume". I resume and it all starts up with no problems. i can't see anything that i can do to change anything. the fan is working fine, from the moment it turn's on.