Ok, so my car (RRS MY06 2.7TDV6) has gone back into the garage to look at the suspension problems. Basically the problem is that it keeps going rock hard. They have had it for 3 days and can't find any issues at all. All track rod ends, ARBs, Suspension struts, etc are all ok. All height sensors are working fine. It's had a new sensor recently, and a new compressor. After every calibration, it seems fine for a short while. I've even spent quite some time with my nanocom fine tuning the calibration.
The only thing they have noticed but can't explain at all is that the OSR is always lower than the NSR despite calibrating it properly. They have no idea why this is the case, and it is something I have always noticed. Both rear sensors are new.

They are saying that there are no faults at all and so can't find what the problem could be.

I asked them to check the cross valves but apparently they are fine. Could this be a wiring issue somewhere, but if so would this throw a fault?

I'm absolutely lost with where to go next on this so any help will be gratefully received!!!

Rear corner valve block needs replacing (or rebuilding). Mine did this - OSR lower than NSR - new valve block cured it. The valve block is about £80 and can be done DIY (although I got mine done by a garage). 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Thanks,
I'll give that a try. I don't necessarily think it is a wiring issue as no faults are thrown up and after you play with the suspension settings, i.e. re-calibrate or even sometime raising the car and lowering it, can solve the issue.
I'm doing the rear brakes so will do this at the same time.

The rear valve block lives on the front face of the NSR strut mounting. Access is a little tight. Probably easier from the spare wheel well side. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Thanks for that. I've just ordered the valve and will look at that this weekend!

Make sure it's the correct block. They look the same but apparently they are different internally. The car will still work and be driveable but you might get odd behaviour if you fit the wrong block. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Well all is looking good so far. I received a cross valve, but they sent me the front and not the rear! I therefore decided to change the front and then ordered the rear. Both have been fitted and they immediately made a huge difference. The car feels lovely and very smooth.
I took the old valves apart and some were very stuck. Fingers crossed this has solved all the problems!

Thanks for your help

 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed