Sirion M100 1999 a/c wont work

Just got a Sirion on the road (in Queensland) but cant get a/c working. Green light comes on in switch when I turn it on but compressor doesn’t kick in. Checked if 12V was getting to the compressor but it wasn’t. Found a wiring diagram (from another model GTi I think) that shows an “a/c Amp” - but I don’t know what that is or where that is? Some sort of relay or control unit?? Cant find a relay anywhere but figure there should be one.
Can anyone help?

Has it got gas? I think it won’t kick in unless it has gas.

First thing I’d try is a regas, they are recommended every ,2 years anyway for hygeine reasons

3rd for regassing it and checking for leaks, will need a new receiver dryer more than likely and get it checked for leaks.