I realise that I am horribly cynical but I got the impression Alan Bennett had really wanted to give us the play within this play. The idea is that we are watching a fairly advanced rehearsal of a play which is by turns brilliant and and comically terrible (talking furniture, talking wrinkles etc.). We get pretty much all the rehearsed play but the players are allowed to comment of on the plot and their characters, query the writer or stage manager (the director is away) and are allowed to try out ideas.
I know that Bennett has done this sort of thing before (Forty Years On has an interrupted end of term play at a school) but I got the feeling that he had started to write a fairly straight play about an imagined final meeting between W H Auden and Benjamin Britten and given up. Possibly this is because of the disjoint, I felt, between the good stuff in the rehearsed play and its more humorous even nonsensical parts. It is as if the funny bad bits are a later thought (but not an afterthought). Of course if you just take the good bits of the rehearsed play, you get a captivating fragment but it wouldn't be enough. You need the explanations and the discussions to make it work and they could not have been fitted elegantly into a straightforward narrative.
The only problem, perhaps, with the play within a play device in this case was that some of the points he was trying to make (which were mostly part of the rehearsed play) didn't quite come across as forcefully as they were probably intended. Of course I'm cloth-brained about these things so he would have had to use a loud-hailer at close range to get his ideas across to me. If I had to say what Bennett really meant by the the phrase “Habit of Art” I'd have to hide behind meaningless waffle until you went away.
All the same it is great fun, Richard Griffiths occasionally playing W H Auden was a treat, Alex Jennings as Benjamin Britten was very slightly underused and Frances de la Tour as Kay, the Stage Manager was her usual quietly brilliant self.