Just to finish this off, we did eventually locate the crystal - on a pcb board underneath the transport (no wonder it was hard to see). Anyway, the clock mod has made a significant difference to the sound and picture quality (particularly the sound) of the DV8400 (and it was pretty good before).
The clock used was a Millenium (manufactured in Malaysia but available here in Australia). It was significantly cheaper than other options, such as Superclock etc. But really, I would recommend a clock upgrade very highly. It makes me wonder why middle-to-high-end cd and dvd player manufacturers don't use separate clocks rather than on-board crystals - the improvement is really worth it.
Thanks for your help.