It feels like some weird conflict between the two IDE devices but I've
never
come across that before - they are on different channels. I've tried a
different DVD-ROM drive and a different IDE cable, same problem.

Tried HDD as IDE 0 master and DVD as IDE 1 slave, no difference
Tried HDD and DVD on IDE 0 as master and slave, no difference

All very strange!

Cheers, Rob.