I wonder if you could help me I am at my wits end with this fault.

I have almost new air bags all round.
I can find no leaks in either any pipe lead or connection to and from the bags.
I have put new seals in the valve block and re-checked them again.
Compressor is fine.
I have done all the checks detailed in the technical area.
The car is doing this all the time now.

When I leave the car the front drops within hours.
If I leave it longer the rear lowers itself (probably just to level itself)

When I start the car the rear will rise virtually straight away
The front passenger side will rise after a few minutes with the front right following after a few more minutes, leveling the car very slowly.

Any ideas, just about to thrash the car with a large branch!!!

Rear always rises first followed by fronts. If the tank only has enough air for rear bags, then the fronts will be slow while the compressor tops up the tank as well. Sounds like you do have leaks somewhere, followed by self-levelling doing it's thing. These together will wear the compressor, so even though it seems fine, it's probably not 100% so the tank is slow to fill.

One evening after switching off, unplug the timer relay under LH seat. Then see if it still drops. Repeat for High mode.
If it stays up with the relay removed, then the airlines & bags are probably ok. If it drops then you need to check bags, lines & valve block more carefully.
If the tank is losing air, (hence not enough air to fully re-inflate in morning), then it's either tank itself, tank airline or NRV in valve block.

It's also possible for the valve block to leak internally and exit the air out the exhaust port. In this case the leaks are hard to find. Replacing the exhaust silencer with a gauge overnight might show something.

Bottom line you have leaks somewhere, so the relay method is the easiest way.

After checking and rechecking all bags, pipes and valve block, I finally decided it was def the valve block so took the plunge and got a secondhand one from Emmotts.

It arrived this morning and I fitted it at lunch.

Oh S*@t! - it won't rise!

Then my mate, who had ambled up the road to talk to me and gloat about his new TDV8, said, "should this not be plugged in?"

He had distracted me with his gloating and the plug in question was just resting where it should be but not actually connected.

Once connected it all came back to life - leaks gone.

And then it passed it's MoT too!

Happy Days!