Cooker Hood will not switch off?

Hi,

I have an Acorn Cooker hood which has suddenly refused to switch off. The buttons themselves still seem to press down correctly and 'beep' when done so but the fan continues to run. Then, at random points, the fan will stop but then starts up again a few minutes later. Lights switch on and off normally.

Does anyone have any ideas what could be causing this?
thanks,
Andrew