Microwave-Grill Oven - H5972NOGB/01?

I have a Microwave-Grill Oven - H5972NOGB/01.
When I use the oven part of it - the temperature doesn't work properly. It starts to heat up - then stops. The only way to make it heat up properly is to maintain constant pressure against the door of the oven.
Although you can take the door out of the main oven - you can't on this one I don't think.
Does anyone know how to repair this please.
Thank you