An increasingly important forum for accountability is the Departmental Select Committee System in the House of Commons. Backbench MPs are appointed to committees which monitor the work of a specific department. The appointment is for the whole parliament, though members may move on early - sometimes because they are appointed to one of the frontbench teams; or another committee; or for any other reason. Chairs of the committees meet regularly (as the Liaison Committee) to question the Prime Minister.
Unlike parliamentary questions, there is an opportunity for in-depth questioning of a witness.
There is an excellent film about the committees which was made for the Parliamentary website.
There is a very useful report from the Liaison Committee looking at the work & effectiveness of the committees. It is available here.