Dropped it off this morning at the local Land Rover specialist (it was booked in for a new rear prop already), so I asked him to look at the other issues. So far it's looking like an ABS issue causing this but he can't get his diagnostics machine to talk to the ABS or Air Suspension. No fuse issues, nothing obviously wrong. He's got a couple of ideas on what might be causing it (either a knackered ABS sensor or the ABS ring has expanded and taken off the tip off the sensor) so he's going to press on with it.