Renault Laguna aircon not working

2005 Laguna 1.9DCi Aircon not working. (Aircon, not climate control)
Had gas charge checked to start with, that's okay. Noticed that compressor clutch does not engage, checked for electrical current to connections, there is none. Also noticed that fan at radiator runs on momentarily after stopping engine as if air con is switched on without it being on. (Button)
Took it to my local garage who also does aircon, they carried out various checks, ascertained that no current at compressor clutch, found a large fuse under the bonnet had blown, replaced, no luck. Checked earth connection at compressor clutch, no earth but by giving it a false earth he could get it to run.
Consequently concluded likely to be safety switch through which the clutch gets its earth. (High pressure?) Part cost approx £80. Switch fitted, aircon ran but stopped after a few minutes.
Disconnected/reconnected battery to try if that may re-set the system, aircon ran then stopped as above.
He was told also that the orifice in a connector may be blocked by sludge? gassed down, checked connectors, re-gassed, no luck.
A small switch in the compressor was the next attempt, part cost approx £12. Switch fitted, no luck.
Maybe should have gone to Renault but we are where we are. My trusty old Laguna (174,000 miles) is still needed until later this year and we also have to go to France twice so need cooling.
Can you suggest anything please? Desperate!!!