Caple CR9200 Dual Fuel Cooker not heating up properly

I have a Caple CR9200 cooker and the main oven isn’t heating up properly – food is taking over twice the usual time to cook. I am pretty sure its a problem with the thermostat as the fan is working and the oven is heating up – did remove the inside back panel to check the element and all looks in tact.
Can anyone advise if replacing the thermostat is something i can do myself, i am a keen and competent diy-er. If so, any advice as to how to locate and replace it and also if a generic replacement is as good as the official part.
Many Thanks