So, an odd day.
Having been through all the connectors, male and female, I could find nothing wrong and everything was nice and snug fitting.
I reassembled and refitted the unit and plumbed it back in.
I started the engine and left the driver's door open to let it fill the reservoir. With that done I opened the drivers window and shut the door. It then raised the car to the extreme 'grounded' (high centred?) ride height, so I got in and sent it to Standard height and it wnet to standard height. Hurrah! Drove it round the yard a tiny bit and all seemed well, parked it and it went down to access height with it in Park and Handbrake On. More Hurrah! I closed the drivers window. And it went up to the extreme 'grounded' ride height with the top indicator flashing and is now steadfastly refusing to go back down...
ARGGGGGGHHHHHH!
I have collected a second complete Valve Block and Pump unit (the one from my ebay link) which I got for 80 quid. Am going to go through the connectors on that now...
The only thing I can think of is perhaps I have a fault in the 'grounded' vehicle sensing system which is overiding everything when it is triggered, I did read in the LR System Information Document that: 'The system will default to extended ride height if the system is unable to lower a sensor's bit count for any ten-second period' Could this be my smoking gun?