Fixed-term Parliaments Bill

I was elected under the law as it then stood, and I expected that the length of term in place at the time would apply. I also expected that any Prime Minister would make decisions in that context. As well as my question of the relationship of the electorate to the House of Commons to the Executive, there is another that hangs over this entire argument: why do we need any of this? What improvement does it bring to the current position?