Well following your advice we had another look at the sensor and yes we were looking at the wrong one! lol Although we did check the suspension sensor previously and it seems to be working although it still seems high at the back.
We looked at the pump which whirs when the arm is moved up or down, the vac pipes don't seem to have a leak and nothing seems obviously wrong. Only thing we could think of was that the sensor maybe packing up? When the car is running the suspension raises a little and then goes down and then back up? Its still too high and it rolls all over the road and the back end keeps stepping out :-/
Any ideas whats causing the problem? I called the mechanics at Vauxhall and they couldn't 'remember' enough about the self leveling to say what it was over the phone and wanted to see the car. So we are stumped

Any help would be appreciated.