Drove it to my Immunisation and Uniform Appointment today.
Got two new holes in each arm and a nice suit only to come out to the car and find the NSR corner sitting low

Fortunately, it pumped back up, so it isn't permanent, but the Airmatic is obviously faulty.
Tested for leaks with soapy water and could not quite decide where the leak is... Not from outside of the valve block, but possibly the NSR airbag or line.

Fortunately, it doesn't have full Airmatic, just the rears, and also fortunately, Mercedes did offer a couple of models of the Alabama Mumsybus without any Airmatic. So took some digging, and I have managed to locate a pair of rear springs for it.

May also require a couple of bits and bobs to get them fitted, but they locate into exactly the same positions, so will be a permanent fix for significantly less than any individual Airmatic parts.