OK. I think I've finished the redesign now. A big improvement over the original (default) template, I think you'll agree.
The site also now has a spam filter (thanks, Matt), so hopefully no more unwanted adverts for Viagra. I hadn't really been paying much attention/spending much time on my weblog, as I've been busily beavering away on my program, however this entire spam incident increased my desire to really sort it out.
Anyway, I'm afraid that I haven't validated the HTML (or indeed the CSS). It seems to work OK in Safari and Internet Explorer (the Mac version, at any rate), which is enough for me, for now.