can anyone identify the faulty part on my Diplomat range cooker?

Diplomat ADP 5426 model no-050567009. I have a top oven/grill and when I switch the power on at the mains the top oven comes on although the selector/temperature switch is still in the off position. It can go off of it's on accord and usually does so after a little while if I use the bottom oven but not always, particularly if I use the grill.
I have replaced the grill element recently. This problem existed before the new element was fitted Can anybody tell me which part is faulty?