Martin - try searching for City Runners, I think they're based around that area.
Annie - I second Clapham Chasers

I've only been a member since November, but they're a friendly bunch and the marathon training groups have been a lifesaver for me! I've also run with Sweatshop before and they're great too.
The thing I noticed when I was looking for a running club is that the name of the club doesn't necessarily mean that's where they meet (i.e. Clapham Chasers are really based in Battersea, Herne Hill Harriers are based in Tooting). Check where they actually meet before you dismiss them for being too far away (as I did!).