Thanks Weasel

Unfortunately not that simple. There is no leak. Everything's been pressure tested at least 6 times by local BMW dealer and independent specialist.

It seems as though it's an intermittent control issue.

Ride height sensors have been replaced all round so it now seems as though I may have to bit the bullet and change the control module £760 although from the BMW diagnostics, for the errors that are being thrown up the BMW manual suggests changing the compressor and control module.

That's one expensive repair to try to eliminate a fault that might not actually lie with either part

Thanks anyway Weasel