Thanks for the advice. I think I have less trouble passing the interview (Google — I hope — is the case where false negative has happened) compared to getting there. On the HN I read a lot about smaller companies looking for all-rounders and I try to position myself as such when I apply. The trouble here is that I see very little positions of that sort being advertised. Startups seem to like RoR and Django juniors the most. I guess, networking is the answer here, as you point out.
Finance industry in London is very insulated (i.e. people never leave banks except for other banks because of the huge salary difference banks able to offer), so networking is a hard thing to crack quickly. I'm working on it though, so at least this I do right.