Fiftieth birthdays are traditionally a time for an extravagant celebration and taking stock of what has been achieved. The National Theatre of Great Britain, which reached five decades on 22 October, is combining both in a gala performance staged at the NT on Friday and Saturday, 1 and 2 November, with the latter performance broadcast live on BBC2. This retrospective event coincides with a key moment of looking forward: the announcement just before the fiftieth that Rufus Norris will succeed Nicholas Hytner, from 2015, as artistic director. This results in a double invitation to look at what the NT has achieved and where it may be heading.One complication of the anniversary is that the theatre in the form we now know it – as a three-auditorium complex on London’s South Bank &n