The only process to resolve the fault seemed to be to keep changing components in the air suspension until the problem was solved. Having looked at the prices of the various pieces of kit and conscious that this is a 9 year old car with 73k (albeit one I particularly like), this just did not make economic sense. At this point I consulted the forum again and decided to follow Raj down the KW Coilover route. I want to keep the car for a few more years and was keen to resolve the suspension issue once and for all.
Raj has been fantastic in sourcing the kit for me and fitting the various pieces. I got the car back at the end of last week and I have to say I'm impressed. The ride is definitely better (the air bags definitely lose flexibility with age)and I like the feel of the handling.
The car is riding slightly lower but not noticeably and I'll get an alignment done in a week or so once the suspension has settled fully.
So big thanks to Raj!

I'll get round to selling the air bags, struts, reservoirs and the compressor on ebay but if anyone wants any of that kit do please pm me in the next week.