Well well.... the garage quoted my a min £150 for one rear sensor and one hours labour. Or it could be the pump and I would be talking £400 upwards.
After some research on the site I looked at the fuses in the boot. Located the fuse (30amp), pulled it out to find it had blown. Replaced the fuse and what a nice sight to see the rear suspension rise up to the correct ride height.
I guess my message here is, just spend 30mins researching on great sites like this one before you take it to any garage and hand over your money

2002 prefacelift X5 4.4i sport V8, Black with black leather & silver trim, 20" alloys, comms pack.