Hi folks, thanks for the replies and apologies for the delay reply.
I went to the car today- it's a bit (!) warmer, and still no compressor kicking in.
Giving a +ve feed to the compressor made it kick in.
Bypassing the pressure switch made the compressor kick in.
Guessing I've either got a duff switch, or the gas has escaped since fitting it a year or two ago
(I know where my money is!)
Guess I'll book it in for a pressure test sometime soon! (As soon as it's not SORN any more and I'm back to needing it!)
Strangely, I didn't seem to have any codes. The blue AC led went on and off in a sequence I struggled to interpret, but I think was 1-1-1... I did take a video if anyone wants to see?