Thank you, gentlemen, for your prompt responses.

I have been running, when it worked, with the exhaust silencer off, and the occasional gasp of air did not trouble me at all.
I was aware of rears rising first on inflation, as stated above, so the nanocom reports that correctly. So why report both inlet and exhaust open at the same time?
I first replaced the ECU, and when this had no effect, I replaced the driver pack.
Unfortunately, the nanocom will not open individual valves, but only command a change of height, as well as reading / calibrating height sensors etc.
However, with the valve pack on the bench, and also in the car, by applying 12v to the appropriate pins in the disconnected C152 connector, I can activate all the valves, and hear them clicking - driver pack OK ??

What I failed to mention in my original post earlier, was that fully serviced, tested and reassembled, it is sitting on its bump stops - full tank of air, ride height sensors in range - and when I switch on the ignition all the air is lost.
This has me completely mystified

Another EAS chestnut, I'm afraid. I have searched on many forums, and copied and printed much advice, to form my own EAS rescue pack, but I am still stymied.

Whilst driving, the car would almost always (except for the MOT journey!) jump to extended height. The next morning, when switching on, the ECU would recognise the error, and lower to normal height, only for the blasted thing to jump up within a mile. I suspected the height sensors.

I have renewed all four height sensors, and nanocom reports height values within three bits side to side - so OK so far.
The compressor will charge the reservoir to 10 bar within several minutes, and hold that for up to a fortnight, until I switch on, when all pressure is lost.
I have very carefully replaced all the seals in the valve block, and soldered all the wiring in the passenger footwell.
I have replaced both the ECU and the driver pack with used "known working" parts, with no change in behaviour.
I cannot think of anything else to check.

Nanocom reports no faults, and appears to read correctly all data from the ECU
However, when reading the valve settings, it shows both rear valves open, and fronts closed initially - OK - but both inlet and exhaust valves open - surely wrong?
What can this mean, and what might be the remedy? I would be very grateful for an insight since I have tried everything I can think of.