Well I didn't find the exact solution I was looking for, but the one I
have found is better. Huzzah etc.
So FYI, here it is:
http://www.spampal.org/
It's a free program that acts as a proxy between your email client and POP or IMAP servers.
It filters spam based on public black/white/ignore lists, and includes a plugins facility.
AND...
One of the plugins linked to from it's homepage is a Bayesian filtering one, so I've now got a solution that does what I was after in the first place (the Bayesian filtering) and a bit more into the bargain. So far it's worked pretty well - it's not accidentally flagged anything it shouldn't have, while still managing to catch the 60 - 80 odd spam emails I get every day.
Oh yeah, because it's a proxy it doesn't interfere with other email scanning programs you may have.
For example, I have Norton AV which scans email for viruses as it's downloaded, so the chain now goes Email server -> Norton AntiVirus -> Spampal -> Email client.
Nifty.