Udash - THANK YOU. Very helpful. With a 16L reservoir, then i can see how it should work without the compressor running for much of the time.
Just such a pain in the ar*e when the dealership give me the usual "...you are imagining it..." or "compressors are noisy Sir!" Land Rover addict

So today i dismantled the acoustic box and rearranged the exhaust from the compressor. And i think i have sorted it.
As far as i can tell, the exhaust was impeding the movement of the compressor which made it sound louder because the natural vibration of the unit was hampered by the pipe being in the wrong place.

Not sure if anyone else has ventured into the acoustic box? The compressor is mounted on very flexible mounts which seem strange at first and there is a lot of heat.

Anyhow, fuller test drive tomorrow.
Thanks for all the help so far. Land Rover addict

Just in case anyone is interested or has a similar issue, LR have replaced the replacement(!) compressor and all is good again.
No idea what the issue was, but a new unit has dealt with the noise and we have a lovely quiet RRS again.
Next task – winter tyres having been embarrassingly stuck in a car park in Meribel last week. Land Rover addict