Belling Platinum db3 main oven thermostat problem

I have a Belling Platinum DB3 range cooker & the main oven thermostat stopped controlling the heat. I called upon a local repair agents who came in said it needed a new stat & a few days later they had the part & fitted it. It is a Diamond H unit & looks very similar to their 46THC8/J5 being sold as suitable for the DB3. The main oven began to loose control of the temperature no more than a week after the new stat was fitted so I called up the repair shop (their invoice offers a 3 month guarantee on repairs) who came out that day whilst my wife was in. They took the new stat out saw that it was burnt out and told my wife it was another problem & they couldn’t repair it? I’m now £160 poorer & still do not have a working main oven.

I’m an electrical engineer & I’m sure that with a circuit diagram & thermostat detail I can find the fault to call out my repair friends to fix.

Does any one have a circuit diagram for a Belling Platinum db3?
Does any one have the detail of the terminal function for a db3 main oven thermostat?

Does any one know why the center terminal “I” might have burnt out/

Thanks to anyone who helps.