1996 (7?) S320 Limo air-con problem

I have just bought a S320 Limo, first registered Jan 1997 - so probably a 1996 build.

Everything is fine except the air-con.

I am trying to avoid using a dealer - they charge a fortune in the UK.

My friendly air-con man has checked the system and re-gassed as far as he is able. A certain quantity of gas has been pulled in by vacuum. Now he needs the compressor to run to pull in the rest.

Unfortunately, the compressor has been disabled by the safety device triggered by lack of gas. The EC (economy) button is permanently lit.

I understand that a Mercedes dealer can disable this device so that the compressor can be run to finish the gassing process, but they will insist on doing the entire re-gas for a bucket-load of money.

On my previous car - a Jaguar - it was possible to over-ride the equivalent safety feature by pressing two buttons on the air-con control panel simultaneously (a well known but undocumented "feature").

Is there any way round this so that my friend can complete the process, or do I have to go to an MB dealer (and arrange a new mortgage)?

TIA

Geoff