Further questions re my 'lopsided A8'

[Diablo]

Further to my previous post on my D3 3.0tdi’s droopy suspension, it has now come to a point that I need to get this sorted. The current symptoms are:

• The car sits lower on the offside.
• The front o/s corner sits lower than the other three. I suspect it is dragging the rear o/s down with it, as it is slightly lower.
• All four corners go up and down as normal, but the offside always remains lower. There are no warning lights.
• The link bars that control ride height are not broken.
• I measured the ride height in the evening and in the morning and the suspension does not lower itself overnight.

I had the car put on the VCDS and it came up with an 01400 “Suspension level control - upper limit exceeded”. It also generated a 01398 code, which is to do with the central locking.
I have spoken to a main dealer, and the tech said that they would have to do a leak test using soapy water He had no estimates on how long it could take...it sounded a bit open ended, so I am reluctant to hand it over to them without exploring all of the other options.

Does anyone have any idea on the best/most economical way to proceed? Alternatively, any recommendations for trusted indi's, in the Heathrow area, would greatly appreciated.
Thanks in advance.