A million thanks for Seo4Smf. I was starting to regret having chosen SMF because of its weak SEO, but this makes up for it. I set up my site yesterday and have just installed Seo4Smf. I slightly modified Subs.php to replace spaces with - instead of _ , as from past experience Google seems to prefer them.
One gotcha to watch out for when installing is that sitemap.php & urllist.php install with write access for Group and Others, which caused 500 Internal Server Errors (I think my host must have tight security installed); once I had chmod them they worked fine.
Anyway, here's the site -
a BBC discussion board. Probably not much interest for those outside the UK, but feel free to post anyway

Thanks again