I did google that, and the overriding impression is that you don't, but even then I've no doubt the party could engineer some by-election to parachute him into. But I read an interesting article the other day that seems to suggest that Cameron has hit the ground running in his new job and is attempting to drag the Tories back to Edward Heath's stance on the middle east which (i only discovered after reading the piece) was actually pretty good. Cancelling arms sales to Israel and SA.
Here's the article. It's quite a long read and not for a match day but for anyone who wishes to peruse it later.