Hi, the problem has been sorted, it was a low level of R12 gas (refrigerant that destroyed the ozone layer) causing the flashing light.
I've had the car converted to run on R134a (current industry standard refrigerant).
The conversion requires:
Changing the compressor to a Gen 6 compressor (second hand from from TCB)
Cleaning out all the pipework to remove the old oil.
Replacing all the O' rings with R134a compatible green ones.
Replacing the dryer receiver
Changing the 1/4" charging point to a modern quick fitting
Using PAG46 oil and filling and pressure checking with R134a gas.
If anyone wants more details let me know, but it works very well now and should be future proofed.