Actually, we can compare notes now. The replacement original pump I got was also fubar, but in a slightly different way. Having negotiated a satisfactory partial refund with the seller, I proceeded to strip both down, get one set of good parts from two duff units, and then built it back up again with new piston ring and dessicant.
5 hours later, I have a compressor which hopefully will work, but which has cost me about £400 in terms of time and parts. I could have bought the Aerosus
and the Maxspeedingrods units for that
Given that the compressor lives under the car in our salty wet climate and in the case of the Allroad, under the back of a car designed to go off-road, I very much doubt there are many good used units left after 20 years. Possibly parts could be used from other makes where the same type of compressor is housed inside the vehicle, but I certainly won't bother rebuilding one again.
Lesson learned
