It was a resistor fault Ian but you appear to have something else wrong.
The easiest way of telling whether the system has sufficient refrigerant gas is to ask Mrs. Macafee to start the engine and press the windscreen demist button whilst you watch the compressor pulley closely. When the button is pressed you should hear a loud clunk and see the centre of the pulley begin rotating.
I don't need to say "let us know how you get on" because I know that you will!
Simon