A Henley by-election?

Writing on the Guardian website, Andrew Sparrow speculates on what a Boris Johnson victory in the London Mayoral election would mean:

According to his spokeswoman, Boris has said repeatedly that, if he is elected mayor, he will not try to hang on as MP for Henley until the next general election.

It's possible, of course, that he could "change his mind". Andrew Gimson, in his biography of Boris, quotes Conrad Black on how Boris gave "his solemn word of honour" before becoming editor of the Spectator that he would not try to become an MP and on how Boris broke his promise almost immediately.