We have the same problem with this cooker unit. Last time it happened a few months ago the service engineer charged about £70 to remove an accumulation of 'fluff and dust' in a vent somewhere down the back. It's a 10 minute job.
I wasn't present when he fixed it, and my wife wasn't watching! so now I'm faced with trying to work out how to dismantle it - anyone out there have the installation manual?
R Steels, December 2005