Sorry about the comment spam whilst I was away. I've now installed MT-Blacklist, so hopefully the only offensive rubbish and stupid sales-pitches on this site will be my own...
MT-Blacklist seems pretty good; it cleaned up the mass of spam with one click but then unfortunately failed to block a new piece of crap that was posted this afternoon. Having added one of the offending urls to the blacklist I ran the cleanup again and it removed the offensive comment and automagically added the other urls that the comment contained to the list...
I'll write more about it once I've more experience of it...