Hi, yesterday we set about replacing the front coils on the TD5. We run out of time and left the front end jacked up +axle stands over night,
significantly higher then the rear. Finished the job this morning but for some reason the rear air bags now refuse to raise the rear at all. I wonder if the fact that the car was not level over night means that the compressor worked overtime trying to level out the vehicle. If I jack the rear up on the tow bar the air bags appear to be firm and inflated but when the jack is removed it drops right back down. Any ideas? Does the SLS system need resetting? Prior to this no sign of leaks or any other rear suspension problems.

The compressor won't run unless the engine is running and all the doors are closed.
When you were working on the suspension, did you disconnect the Ride Height Sensors? If so,did you disconnect the battery before you started work?
I don't think the system needs recalibrating just yet, but it does depend on what's been done to the rear suspension.
The reason for lifting the back end with a jack under the towbar is to start bringing the outputs from the Ride Height Sensors to the SLABS ECU back towards the expected values forcing the system to then actively adjust the height back towards the proper level.
The air bag might feel firm but the system needs to inflate the bags under control of the SLABS ECU, so the compressor must be run.

We just replaced the front coil springs - no work whatsoever on the rear suspension or the height sensors. Battery was connected the whole time.
With doors / bonnet shut, engine running and left window open I can hear the solenoids clicking but no sound of compressor itself. The rear end stays firmly at the lowest I've ever seen it.
The 30 amp fuse in the engine bay is OK, I've tried swapping the relays #5 & #3 to no avail.
Up until now I've had no signs of the the SLS failing (no sinking overnight, no trouble using O/R mode etc.
I guess it's time to get the compressor out and test with a direct 12v feed.

Did you inspect visually the height sensors to not have one snapped? What if you push the ORM switch?

The actual circuit for the SLS compressor is pretty simple; +12 to the fuselink FL9 (not the 30 Amp fuse), the relay contacts and the compressor motor.
If FL9 is OK, proved by measurement rather than just looking, a quick check of the circuit would be to pull the relay and use a piece of wire to bridge pins 30 to 87 on the relay holder. You should hear the compressor running. If it doesn't then it's either a problem with the compressor motor or in the wiring. Do not overdo this test, the compressor will only stand being run for a short period, duty cycle and all that.
Failing that, as @sierrafery as already suggested, examine your ride height sensors for damage or sticking.