My freezer isn’t defrosting and there is a lot of ice build up around the evaporator. The high temp light comes on too but not all the time. Not sure if its the defrost heater element or perhaps the timer not kicking the heater into action.
Is one part more likely to fail than another? I can get a new heater from Hotpoint for £25. Can’t find a replacement timer though.

If its an old one just replace it, the A rated ones can pay for themselves within a year or two compared to the old inefficient ones (those ones most people put in the garage and use as a beer fridge…expensive beer!)