Air suspension.

I didn't see that option.
I have checked the airline as far as possible and removed the offending bag, it looks pretty ordinary. The smooth, round screw-in, push fitting is a pain in the fundamental.

Go with nanocom in SLABS - Utility- SLS height calibration and there you have functions to raise each part 2-3cm at one push, do it with engine running... if you succede to raise it to even level you can store those heights too cos if you unplugged one sensor or it had a bad contact with battery connected the calibration can be lost then the compressor might not kick in

Thanks Seirrafery, I'll put the new bag in tomorrow and give it a crack.
I've only undone the mechanical connection to the trailing arm, not the electrical plug ot the height sensor.

Air bag replaced, not the simple task it should have been, but I eventually got there.
What values should I be setting for the rear height? The car has not been lifted, nor will it be.

The values shown by nanocom are not the best to rely on them, according to bbs "value of 1 is equal to approximately 1.4 mm between the wheel arch and ground" (so it's an approximation), you have to inflate the bags and measure the heights from ground to wheell arches all around the 4 wheels untill they are the same and store what ever values you see cos the values can be slightly different with even level while your interest is to have a perfectly straight vehicle. I used a two way T-shape bubble level tool(like in the pic) on the middle of boot floor, used two jaks one each rear side to lift the body and stored the heights when the bubbles were both in the middle while the values shown by nanocom were not 100% similar, important is to be on a straight surface hen you calibrate(btw, while you are using nanocom to infate itt doesnt matter if the doors are open or closed. Good luck

Thanks Seirrafery, I've set both sides at 150, a little lower than it was, but that compensates for the 30mm sag on the front springs. I'll be more fussy when the new Bilstien dampers are finally available and I replace the front springs and dampers.
I'm not entirely convinced that the issue was the airbag, as after I finished, the RHS dropped again. I reset it again, then went for a drive, all was good.
This morning I sourced two metres of 4mm airline and a number of push-in fittings, just in case.

If the bag and the pipe are 100% leakless and keeps dropping on that side the problem can be the height sensor or sticking valve, to rule out the sensor you can swap them over and if the other side will drop then you know...just dont disconnect a sensor with battery connected