The Speaker, Labour and the BNP.

During today's PMQs one member asked Harriet Harman about a BNP candidate who had commented that rape is not a heinous crime and compared it to "force-feeding a woman chocolate cake". Conservative MP Charles Walker condemned the comment maintaining that the candidate was not fit for public office. Ms Harman replied agreeing with the condemnation and adding:

The best way to avoid a BNP member being elected to the London Assembly is to make sure that as many people as possible vote for all the other parties.

While I fully support the condemnation and hope people do not vote for the BNP I was left slightly bemused by the exchange. Before Tony Blair left office David Cameron asked him who he would recommend to be his successor as Labour leader, and hence Prime Minister. The Speaker refused to allow Cameron to ask the question. So why was this question allowed? Surely a question directly pertaining to the next Prime Minister is far more appropriate in the House of Commons than one relating to a local election?

If MPs cannot ask questions about political parties during PMQs why was this allowed, and if they may, why was Cameron's not? If anyone has an explanation please let me know.