Thanks for that info Dojj. I've actually read all that when I've been searching for this fault. My mate (Black some of you may remember him) came round today with his diagnostic tester.

On the tester it's saying one of the pressure switches isn't working. Now this has confused me cause as said above the low pressure switch is getting a live and a live is coming out of the switch. When you disconnect this switch the l8ve coming out of the switch isn't there. So this to me indicates that the switch is working??

My brother in law has a diesel mk3 so tomorrow I'm gonna Rob some bits off it to try and get mine to work.
So does anyone know if there is a separate relay for the air con apart from the full throttle one in the engine bay fuse box.