Aah, thanks for that Charlie,
Looks like you hit the nail on the head. I hadn't realised it, but it appears that the drive itself is being 'clever' in order to prevent failed disc writes at a speed they cannot sustain reliably (if I'm reading it right). It'll be interesting to see what happens with the new Maxell discs I have on order..
Thanks for taking the time to look into it for me
Rob.