Okay - so took the Disco into the local dealer today as requested, and gave the technician (funny how they're not mechanics any more!) the full rundown on the issue. Not surprisingly, he rang back an hour later saying "well, according to our test machinery the lights are set up perfectly..."
To his credit he was happy to acknowledge that this meant little in real terms. He did however confirm that to the best of his knowledge there is no way of adjusting the halogens separately to the bi-xenons. He has sent what he calls a technical bulletin to LR Australia asking about this, and I should know the result of that process tomorrow. Will post again once I've heard something.
Thanks for all your info, Caver - appreciate the effort to help out. Graeme - maybe we need to see if we can flush out more owners with this issue, and get lots of tech bulletins going in to LRA. I wouldn't bet on it making a huge difference, but it might just prompt something positive.

