Steamship, Architex - thanks for the replies again
I think it is something to do with the selected speed at which it greys out has been altered.
Once the car drops below 20mph all the suspension options are selectable. I can select them fine and they work.
The car doesn't throw up any error codes.
On a side note, it's quite a complex system the air suspension and it does pay dividends to understand the system well.
I found out that the diesel A8's and the petrol A8's have different compressors. I've manged to source a compressor of a much later A8 petrol with the following part number 4E0616005H.
I will rebuild this one and it should cure the slightly low output I have with my (tired) replacement.
I've had to get much more involved with the repair than I anticipated but there is some light at the end of the tunnel.
I'll report back once I have done some more work on the car.