Baumatic Electric Fan Oven

Hi

We have a built in Baumatic electric fan oven. Recently it has started not cooking food properly. The temperatur light goes out and there si heat but food takes longer than expected to cook (1 hour to cook frozen oven chips). I suspect the thermostat has gone,does this sound feasible?

I have changed the element before and this is not displaying same symptons, element was only changed 1 year ago.

If it is the thermostat, any guidleines on changing it?

Thanks

P-Bro