Cooker tripping mains fuse HELP !?

I have a problem with a Baumatic BT2385. Grill with fan trips main fuse, sometimes immediately, sometimes when it has been on for a short time. Engineer here yesterday replaced function selector switch. Went to pre-heat oven in evening, on fan oven, warmed up for 10 mins then it tripped everything. It is the intermittent nature of the fault which is driving me mad. Oven can work perfectly for weeks even months then (after I tried to use grill for second time in life of appliance) trips board again.

I had electricity checked out and the sensitively of the trip was tested and found to be within unacceptable range. My dilemma is whether it is an appliance fault or my electricity. I can't afford a new cooker but first engineer a few months ago replaced switch (same one I think as yesterdays engineer) the fan, an element and I thought it was sorted but same problem is back. Do I get rid of the cooker or start with another electrical check ? Help Christmas will be a nightmare if it is still playing up.

Incidentally I just found another person who posted on here last month same fault on a Baumatic. She received no answers but had been advised elsewhere both elements needed to be replaced.