I haven't actually been yet because you have to jump through a few hoops first, but I've finally got off my butt and written off for tickets today. You have to write to your local MP for them. I got the address from here - http://www.parliament.uk/mps-lords-and-offices/mps/
The earliest you can book for at the moment is September -- four months away. So I will let you know how long it takes to get the tickets back.
It seems very wierd that you have to bother your local MP for Big Ben tour tickets. Haven't they got better things to do?