How to fix oven problem on Creda Europa 48145?

Can anyone help me? I have a Creda Europa Solarplus, model no 48145 (14 years old). The problem is that the main oven sometimes - well, about 50% of the time - does not heat up properly. The red light to show that it's not up to temperature illuminates OK, and the fan seems to be running, but it may get tepid, or warm, or hot, or it may work perfectly. Switching it off at the mains ,then on again, sometimes works.

I'm good with spanners, but that there electrickery is not my forte, though I can use a multimeter OK. Can anybody give me any clues on where to start looking/prodding/bodging? Please help....