Air suspension (yes again!)

My 2001 Disco 2 has developed a problem with its suspension - I'm not alone here am I? It sits slightly lower than usual at all times - maybe an inch only. When loaded or towing its drops further down so that the top of the rear tyres are above the wheelarches but not usually any lower. It does not drop right down when the engine is off so I'm not convinced it is soley an airbag problem. When I try to raise the rear suspension (just when checking not when loaded etc.) the light on the dash flashes and it pings for ages but it doesn't actually go up at all.
I recently had ACE problems and had the pipes replaced - could this have dislodged anything?
Any ideas out there?