Freezer problem:- defrost heater stuck on, why might this be?

The defrost timer may need a bit of adjustment. The defrost timer should have either a thumb wheel or a slot for a screwdriver on the front of it, and turning it slowly anti-clockwise until the compressor starts, maybe all that is needed. You can identify the timer by the shape of it. Most are a rectangular shape with one corner cut off ( a bit like this :

If you give the cam a turn and the compressor starts, then in a day or two it fails again, this means that the little motor in the timer has failed or is failing and the unit will need replaced.