Ok people I've managed to fix my AC problem! It was the relay!
As I suspected, the pressure was reading high because the compressor wasnt running. After reordanising the relays into the right order
(previous owners / mechanics had jumbled them up like a jigsaw) I managed to carry out the bypass to see if the compressor would run. It did, and the pressure gauge then dropped to a more respectable level. The AC then pumped ICE cold air into the car!!!
To be sure it was the relay, I then swapped the relay with the relay that runs the fuel pump, and what da ya know, the car wouldnt start!
I also measured the voltage coming into the AC relay socket when the system was ON and it read 13v.
I opened up the relay and found evidence that water had got inside and corroded the contacts. This must have happened when the relays had been left to hang upside down instead of being secured in the housing (be careful to make sure your relays are secure - if you have the V6 model, check out my attachment to see how they should look if they are in correctly).
After cleaning off the rust that had formed inside the relay I was able to refit it and now I have a fully functional AC system once more!!!
So if your having AC problems, and your fuses are OK and the pressure doesnt read too low, check your relay!