Lambeth Council has just made available statistics which show that, although the overall turnout was only 20%, the number of postal voters who voted was much higher at 46% (there were 1023 postal voters out of a total electorate of 11,236, 484 of them voted). In one voting district the turnout was only 16%.
This must mean something, probably that it's easier to vote from home than to go to the polling station or maybe that those who go to the trouble of registering for a postal vote are more interested in voting. The Green Party agent said that they had sent their manifesto only to postal voters.
The trouble is postal voters get to cast their vote before the election campaign is over, which means they can't take into account any last-minute developments.