So I fixed the underlying cause of my problems by looking at this non-bug on the kernel bugzilla; now my machine powers off properly. A suggestion from Nicolas Brouard.
I had to run xfs_repair against / (/dev/hda7) on tom (from the FC2 rescue CD - loving the new mirror.ac.uk for download speeds) to completely fix it.
Thanks to Nik Borton for pointing me at one direction for a possible cause. I love blogging and I love bloggers.
Hmmm, a 'nic', another 'Nicolas' and a 'Nik'. A whole world of ni[ck]*s.