Reply #1: From Wikipedia

[T_i_B]

http://en.wikipedia.org/wiki/Prime_Ministers_Questions

Backbench MPs wishing to ask a question must enter their names on the Order Paper. The names of entrants are then shuffled in a ballot to produce a random order in which they will be called by the Speaker of the House of Commons. The Speaker will then call on MPs to put their questions, usually in an alternating fashion: one MP from the government benches is followed by one from the opposition benches. MPs who are not selected may be chosen to ask a supplementary question if they "catch the eye" of the Speaker, which is done by standing and sitting immediately before the Prime Minister gives an answer. The Leader of the Opposition is traditionally the first MP from the opposition benches to be called after the first question (whether it comes from the government or opposition benches), and the leader of the next largest opposition party is the next MP to be called from the opposition benches.