So I managed to revert the kernel back to the base version (2.6.22), and had that booting (until it hit the MTD issues, etc.). And it ended up being quite painless to patch to 2.6.23 A relatively small number of files that needed their rej files checking. Most we're VC changes to file header comments. A few we're deleted files.
Unfortunately it fails to get anywhere booting :( The zImage decompresses fine, but halts straight after that. I'll probably take a step back from that, and incrementally apply the 2.6.22 patches first. Narrow down where the boot discrepancy starts. Rather than trying to run straight to 2.6.23
Finally, for today, I've decided to blitz the laptop. The Ubuntu community docs describe a way of installing from a USB stick. I've tried the initial install of this and it works fine (it's a network boot).
As I type the install is setting up the base system. Existing partitions have already been destroyed :O
Tomorrow I should then have a brand new Ubuntu laptop to play with and setup (according to the Wiki) :) Plus 40GB of space to play with, rather than the FAT limited 4GB home space :S