Disco 2 Air Suspension Not Rising: Sensor or Compressor?

I've done a bit of searching and reading so I think either my compressor is gone or one/more height sensors. How can I tell which?

Symptoms are as follows...
On starting engine (with all doors closed) the TD5 D2 does not appear to rise from parked height. It's not down on the bump stops just not coming up the 20/30mm I would expect.
Pressing the 'off road height' button I get the chimes and then the flashing light but it doesn't rise and continues to flash. Once I get to 18mph I get the chimes again and the flashing continues until I switch ignition off.
If I don't press the off road height button it's fine driving round town (maybe a bit low at the back). No lights come on.
If I go over a bump (speed bump, kerb etc) I can hear what I think is the compressor running for a second or two.

The sun's out now so if there's anything I can check from inside the car or outside, I'll give it a try. I can't jack it or get under it.

Hi Higgo,
If you can't hear the compressor when trying to go into offroad mode (with all doors shut) then start by checking any fuses and relays etc to eliminate an electrical fault. If there is anyone near you who can plug in a tester (Nanocom/Hawkeye etc) then they can help even more.
HTH,
Griff