Thought I would give a final update on this thread as the suspension issue is now sorted. Seems that there was a wire within the connector on the top of the damper strut that was damaged, causing the fault. Must have been like it for some time as the suspension didn't seem to have been disturbed when I first got the car. As mentioned above, when the AVS system detects a problem with a damper it will position all other dampers to match, ensuring that stability is maintained (doesn't give any warning lights etc. on the dash however). Car drives exactly as I would now expect and as a few of you have stated, the AVS system is very much noticeable between settings. So pleased, totally transforms the ride and enjoyment from the car.
Would also like to mention how good Lexus Newcastle have been, I'm very impressed with their service levels and technical ability and as such would whole heartedly recommend them should anyone be looking to use their services.