Unable to adjust the water temp on a Creda Jet Spa?

I am finding that the temperature dial on my Creda Jet Spa make no difference to the water temperature.

My water pressure is fine so i don't believe that is the issue, which just leaves the possibility that the heater unit has been wired incorrectly.

There are 4 connector points on the unit, but i have not been able to find any details about correct wiring, can anyone tell me which points the wires should be connected to as i am unable to locate any wiring schematics online.

Thanks.