The pressure is currently at about 150psi and I'm getting the 'air suspension inactive' message. I have read that the piston rings on the compressors can suffer over time and this could be the problem. It is only £15 for the repair kit from eBay and it looks easy to fit. I'll give it a go and report back...
In the mean time, other than a complete compressor failure, any other ideas on what it might be? Suspension is at normal ride height but won't move due to this error. It is a 2002 Vogue 4.4.

MC