Could be lots of answers to this one. Heater element, Heater relay board, but the one you can try and is most common is the overheat thermostat on the heater element. Isolate the electrics and remove the back panel about 10 size20-torx screws. You will then see the element, and the overload is a small 20mm diamater part with 2 wires on it. If you have a test meter this should have continuity 0 ohms or there abouts. If it is open circuit this will be your fault. Tip from the trade remove the 2 wires and then remove the overload size 10-torx I think, could be 15-torx then throw the overload at the floor from about 6 inches flat side down. You must be thinking i'm an idiot now??? Why I am telling you this?? There is a small concave disc inside. This has been changed to convex by it overheating and when you throw it at the floor you can reset this back to its original state. These are somtimes refered to as SOD's (single operating devices)
As I have said not easy one to diagnose but this where I start first.
MC, October 2012
