Intute is covering the London Mayoral election, as well as the UK local elections through its social sciences blog. I’ve only taken a quick look, and the entries so far provide links to some very useful background information on candidates, the elections themselves, policy/discussion papers and alike.
I suspect that the coverage will remain largely gateway-like throughout the elections – Intute not really the place for expressing political opinions! Keep an eye on the blog though because, like the rest of Intute, it really does look like providing links to a set of comprehensive resources.