To answer my own post - in case it can help anyone else trying to service the ‘unserviceable’ solenoid crossover valve.
1) The air feed hose from the compressor is a push fit which should release if you push the raised shoulder where it enters the valve block. Mine was seized so I damaged the 8mm pipe pulling it off.
Fix was to cut and fit a stainless steel 90 degree elbow push fitting with a short piece of new air hose.
I removed the valve block due to only one rear strut raising after fitting a new front leg.

2)Vehicle not lifting was caused by 3 of the 4 one way valves being seized due to water damage (the air tank had about half a litre of water in).
All internet searches came back with the individual valves are not serviceable on the early valve block, just seals. So I thought ok I’ll get a new one……. OEM = nearly £700! And aftermarket were £250 so that idea went out the window.
Fix was to disassemble the block and spray each valve with electrical contact cleaner while pushing the valve up and down with a small jewellers screwdriver. This eventually freed them off and then I sprayed each with silicone lube before reassembling with a new seal kit from 4x4airseals.
Total cost was under £40 ….. £19 for the seals, £10 for the hose connector and pipe, and £8 at Halfords for the sprays.
Vehicle now raises and drops on the button. Hope this helps someone in the future.