Air strut tests and when it was submerged in a water bath no leaks could be found. So I guess that rules it out then

Thanks Weasel for your info - I have relayed it on to my independent BMW specialist and although your theory is interesting he does not know what solenoid block you are referring to.

He says the valve block near the compressor has only one circuit and that leads to the accumulator under the driver's seat (right hand drive). There is then a valve block further forward under the driver's pedals and this is the one that he says controls the ride height at the four corners. Is it this you are referring to? Your post said near the compressor in the rear.

Can you expand please as it's an interesting theory which may explain my issues.

The faults that keep recurring are; a cam bus error and constant request for filling

Thanks