It has to be taken into account the question of whether Sir George Young's appointment to take over Alan Duncan, where he will have an immediate effect on the expenses rules and the like, is a bid to unseat Bercow in the long run. We all know the stories around the Tories dislike of Bercow and how more than half of their core supporters would rather support UKIP's Nigel Farage than vote for Bercow.
Sir George Young was the Tories preferred candidate to replace the previous Speaker, Michael Martin. Maybe the Tories want to off seat some of Bercow's role and have their preferred candidate more kindly placed to influence discussion around expenses. Or maybe they want him to get more used to this type of political discussion ready for when they try and replace Bercow?
Stating why he didn't do it sooner, Cameron said that he had to give himself time to think about it, which again is interesting, as makes a stronger case for Young being a strategic decision.
This is purely speculation, but it is interesting to consider how much this had an effect on the decision.
Blondie - Rapture
1 day ago