Hotpoint FFP187BP Fridge Freezer defrost system

[nowthen]

The evaporator is not defrosting, so the freezer compartment will not get down to temperature.
I would like to test the heater for continuity and test the thermofuse. If the thermofuse had 2 wires then it would be obvious how to test it for continuity but it has four wires? Does anyone know what these four wires are? (grey, red, Black, blue.)
Also, does anyone know where the clock is that controls the defrost circuit?
Thank you in advance
Defrost heater.jpg
Thermofuse.jpg

[monkey spanners]

No idea on the wires, but wonder if its a combined defrost termination stat and also fan delay stat in one, hence the four wires. Probably not got a clock as such more likely the defrost is controlled by a circuit board that runs everything.