Thanks for your response. Had a careful look into the machine and could see the front fan working intermittently and the power supply fan working - and I think the graphics card fan working. However, there's just a large heat sync block over the motherboard - doesn't look like there's a fan there - although there could be one under it I couldn't see.
The core temp is still running at an average of 70 degrees centigrade - and sometimes goes over 80 degrees. I'm using "Core Temp" version 1.5 to check these figures.