OK, time to update on this AC issue.
The new pressure switch eventually turned up, it can be fitted without loosing any gas, there's a valve underneath which it opens as you screw it in.
It looks like the AC pressure feature in fiatECUscan doesn't work, as the new one also reads 0bar, so this seems to have been a red herring.
We started tracing the connections back from the compressor - the relays in front of the battery were all mixed up - the AC one turned out to be the one with a white plastic cap over the main block, with very small holes that we couldn't get the multimeter through! As they unclip, we could check underneath (I also rearranged them to match the diagram on the cover)...
I found that the supply side of the relay had no power, but the ECU / switch was switching. I don't know where the power to the relay should be coming from, the 40A fuse at the back of the battery is OK.
So, in the end we simply ran a wire from the battery to the relay, via a 10A fuse.
The AC now works fine, and switches on and off with the original dash switch.
On the subject of the pressure switch, if you loosen it with the compressor running, it knocks off the compressor...
Anyone want to buy a new pressure switch???!!!
The AC is now very cold, soon after startup.