Iraq: Future Strategic Relationship

During the Queen's Speech debate on foreign policy, the Foreign Secretary confirmed that it was not the case that every single British troop would have to come home before an inquiry could be held. The Prime Minister said in his statement on 18 December that by 31 July this year, there would be fewer than 400 British troops remaining in Iraq. If that is the case, will that be the trigger for an inquiry? In the Secretary of State's estimation, how few troops must remain before an inquiry can be called?