How do you remove the 10mm securing nut on air con clutch to check and remove shims if required.
Just fitted new radiator and regased, also jumper wire in pressure switch and still nothing

Thank you

There are 3 X M6 tapped holes in the pulley assembly, put a couple of bolts/studs in them and using a screwdriver or similar to stop it from turning, release the nut..which is a normal R.H. thread

Are you getting 12V at the round connector on the compressor (Clutch leads) ?

Before removing the shims, you could put 12V to the two clutch wires with the round plug disconnected just to confirm whether the clutch energises o.k. If it does, you have a wiring problem.

Removed plate this morning no shims in there, gap must of been around 10mm. Cleaned it up and replaced it, now its tight too plate and on is on all the time. At least I have nice cool air.

By "Mechanically" holding the clutch engaged, the compressor will not be protected by the pressure switches, the system will overpressurise and probably blow a hole in the Condenser...they are a weak point on P38's at the best of times. The gas and oil will be lost and the compressor will suffer terminal damage including disintegration.
You should have around .020" gap at the clutch plate when disengaged and 12Volts is required to energise the coil on the electro magnetic clutch. If the 12V signal is not there or if it is and the coil has gone..those are the areas which you should be looking at.
I had a compressor disintegrate on a Carlton years ago, luckily it was driven by it's own belt, which came off in the process. The P38 with it's single serpentine belt does not afford that luck, you will lose power steering, cooling and the alternator drive all at once.

Reset gap to approx 20 tho. Definitely an electrical problem