De Dietrich Fan running?

There is a cooling fan which runs on after use of the oven on my FM4827 De Dietrich cooker.

It wo'nt stop running. It used to run a short time after cooking until the heat had cleared and the oven was cool. Now it runs on and on and on. I have to switch the master switch on the wall to stop it.

Any suggestions? Is there a thermostat somewhere not switching off?

Peter
February 2011