How can I fix my Diplomat ADP5603 oven?

I have a Diplomat (MFI) ADP5603 gas oven and the (electrical?) fan of the grill will not turn off (presumably it's the thermostat?), just keeps running, so have turned off the supply. Can anyone tell me where I can find a picture of where the thermostat is on the appliance? Also how expensive it is likely to be to get fixed if I buy the part myself before I call in an engineer?
Thank you.