Interesting switch in the detail of the new legislation proposed by the Liberal Democrat/Conservative government. The fixed term can actually be thrown out before the five years are up. But not on a simple majority. You have to get 55% to get em out. So Gerry Fitt and Frank Maguire would not have been able to buck Jim Callaghan out in favour of what turned out to be the Thatcher era. David has the detail:
The parties agree to the establishment of five year fixed-term parliaments. A Conservative-Liberal Democrat coalition government will put a binding motion before the House of Commons in the first days following this agreement stating that the next general election will be held on the first Thursday of May 2015. Following this motion, legislation will be brought forward to make provision for fixed term parliaments of five years. This legislation will also provide for dissolution if 55% or more of the House votes in favour.