Hi MZ, that's very helpful thanks.
When rotating the disc by hand (wheel off) it only just grabs it, with the wheel on, the momentum caused by the weight of the wheel makes it barely noticeable. I suspect the stationary nail head would not pick up such a small 'out of true' warp. Neither would my eyesight!
At speed, driven, it feels like a rumble strip on the motorway - well, 60% of that sensation.
The car needs to go back to BHM at some point for another issue - I'll ask them to check and replace the disc under warranty.
