Is it possible that the system needs a recharge, that somewhere there's a pressure switch that without a pressure is cutting off the compressor?
Yes, there is a dual pressure switch which disables the compressor relay if there's not enough pressure in the system so make sure it has pressure in the first place
 
I can "force" it on using my Lynx

Can you read pressures in the system with Lynx? Would tell if no gas, or if you have gas, a sensor problem. (Bit of course this may not show if a sensor is duff) Could then only be confirmed by physical checking for gas.

If it has lost gas where’s it gone and why?

J
 
Well, update on mine and maybe linked in a way to gjmac s problem.
Although I have now got a cheep multimeter, my dearest wife spotted a 2nd hand Hawkeye locally. Ended up buying this because it was coded for the D2 same as ours.
I was able to check air con relay and also clutch; both functioned normally. Seems to have power at every point from heater switches to relay and clutch.
Speaking to a landy mechanic who is doing my exhaust head gasket, he thinks the gas hasn't been pressurised correctly by the radiator garage i went to. That garage told me to have a electrical fault diagnostic done by their friend in the next unit. They didnt want me around whilst they did the test but said a regas was free afterwards with the unit next door. Now it all sounds a bit fishy, because i cant see anything wrong anywhere. My landy chap said discos can be prone to loosing pressure from a leaky pipe in the front behind the grill, but said a proper pressure check should have picked this up. I don't think this was done when it was regassed. Think I will get my landy chap to check it properly, I'm sure it will work afterwards, even if a pressure sensor is needed.
 

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