Although David Cameron had more selfish intentions when he introduced the Fixed-term Parliaments Act – guaranteeing a prime minister five years in most circumstances – theatre has been a huge bene­ficiary of having 60 months’ notice of the general election. The 2015 poll was the first time that artistic directors could schedule shows with the certainty of them being seen by theatregoers about to be voters.So a revival of Caryl Churchill’s 1976 play Light Shining in Buckinghamshire might have seemed an odd choice for the opening show of Rufus Norris’ spell as artistic director of London’s National Theatre, except that, set in the 1640s during the English Civil War, the drama deals with the origins of British parliamentary democracy. The first act end