If you can have the fault codes reset and check again in a few weeks to see whether it is the compressor, a compressor fault would affect all feeds to the bags, so it could be the exhaust valves if you are just having problems with the front and it seems pretty random. The valves open/close are triggered by the ECU which in turn takes it input from the height sensors. I would eliminate all the suspension components one by one before spending a lot of money.
Here are the people I use for parts:
http://www.lrparts.net/scp/DISCOVERY_3/SUSPENSION_EQUIPMENT.htmlIf your G4 does not have extended warranty, you could end up spending a lot of money without really having identified the problem. Although we have had 6 years of the D3 in the market, mechanics are still not experienced enough to make precise diagnosis - it is a very complex vehicle. Dealers keep changing components until a problem goes away, usually taking several visits and emptying of the wallet, you already knew this. Try posting on disco3.co.uk and seek some info to your decision making.
By the way anyone who owns one of these vehicles and you are thinking of keeping it for a while, I would strongly recommend you get an IIDtool or MSV-2.