bcbc .... thanks for the further advice.

I repeated the boot on the XP machine with nomodeset, and it seemed to work OK - it still reported the 'GPIO ininitialised', but not the two later messages.

It then ran Ubuntu like a dog, which brings me onto the main lesson you have taught me - don't waste time shoe-horning modern software into obsolete hardware.

In that spirit, this 'success' with XP encouraged me to try out the same boot disk on a Vista PC (Dell Inspiron 530), about half the age of the XP machine.
.... it worked fine, with none of the three messages - I'll try (dual) installing it later.

But I may also have a go at dualling Lubuntu or Xubuntu on the XP machine, as you suggested.

Thanks again for all your help.