Justin has all the relevant details. I strongly recommend getting involved. I've just sent mine:
Dear Anne Begg,
In March 2003, you voted against the declaration of war against Iraq. Since then, a number of allegations have been made with regard to the government's conduct in relation to all aspects of the Iraq war. As such, I am writing to ask you to support Early Day Motion 1088 (Conduct of Government policy in relation to the war against Iraq). Given the many allegations, I believe a thorough review is in the best interest of all concerned and hope that you share that view.
If you are not able to support this motion, I would ask that you explain why this is the case.