I have just gone through the whole procedure as written by jeffreyallison. If you follow his diagnostic approach you will be able to determine where the fault lies.

I took the control unit apart and cleaned everything with contact cleaner (Halfords).

I removed the potentiometer gave it a good soak and re-soldered it in.

Re-assembled the unit, re-soldering all the joints as I went.

Tested the outputs from the controller and all were found to be correct.

Refitted it all in the car and it still didn't work.

Replaced the "blower resistor" and all is well.