Hi, we have a Hotpoint EXFL1810P frost free upright fridge freezer, approximately 20 months old. High temperature alarm came on, both fridge and freezer compartments were affected. Back panel in freezer was iced badly. Removed this and the evaporator was totally iced. Did a full defrost, checked drain hole which is all ok. Noticed that one of the thermistors (there are 2 on the top pipes of the evaporator/heater ) was hanging loose. There is a double thermistor fitted to the back evaporator pipe, it was the single one which was hanging loose. I clipped this onto the front pipe? Refitted back panel and turned power on. Within about 4 hours fridge and freezer were both back to proper temperatures (purchased thermometer for each). However temperature started to rise again after about 7-8 days. Evaporator is iced up again. Checked resistance of the heater, across terminal block, and got a reading so I presumed the heater is not broken ? If it's the defrost cycle not working is there anything else I can check myself before calling out an engineer? Thanks in advance.