The sign outside the building reads: National Theatre. Inside, you could be forgiven for thinking that it's not exactly the whole nation that is represented.
The day my group of young adult students watched a matinee performance of Tracey Letts' much-praised drama of family life August: Osage County, the audience they sat among were very different from themselves. In fact, one said to me that she couldn't believe there were so many elderly, white, middle class people in the world!
That shouldn't have mattered a jot. (But then, as someone who ticks two out of three of those boxes - and is rapidly heading towards ticking the third - I would say that, wouldn't I?) Still, an audience is an audience. Everyone is free to buy a ticket for the theatre.
And these days the National prides itself on attempting to draw in as wide a cross section as it can. Nicholas Hytner, the artistic director, is committed to staging exciting and challenging theatre. There is a varied programme of outreach work with children. Many tickets cost only Pounds 10. And ours, worth at least Pounds 30, were offered at a student discount that meant we paid barely half that.
So, as I say, it shouldn't matter. But it did. Several of my students - a typical ethnically diverse south London mix - felt uncomfortable, out of place, unwelcome.
It started with the rustling. Part of the object of the visit was for them to write a review of the play. So I had suggested that they might want to make a few notes.
Quite how the professional critics manage to scribble in the dark on first nights, I don't know. But my amateur reviewers didn't find it easy. At the first interval, an usher appeared. She wasn't happy. There had been complaints, she said: someone was rustling papers far too loudly; someone else was using the light from a phone.
Very politely, one of the students explained what they were doing and why. Sorry, said the usher, it had to stop. The student looked towards me. Heroically, I slunk deeper into the shadows.
There is a cult of silence in the theatre for a good reason. Anyone who has sat through a Shakespeare matinee with an audience half full of juveniles, dragged in because they are "doing" Macbeth, will say amen to that. So if my lot were disturbing others, fair enough.
Act Two was uneventful; at least in the audience it was. On stage there were fireworks - of the sort that only a family drama which lists a fight arranger in its credits is going to give you.
In Act Three the dramatic temperature was ratcheted up still further. There was laughter, tears, shocks and revelations. Many of my students, untutored in the art of Anglo-Saxon restraint, reacted accordingly: when something was funny, they laughed out loud; when something was shocking, they expressed shock.
Some sitting nearby seemed to be more shocked by the students' expressions of shock than the events unfolding on stage. Heads turned. The lights may have been down, but you didn't need much illumination to see the disapproval.
Clearly this was a clash of cultures. But the key question must surely be, which, if either, should be the one to give way? Should those brought up outside the culture of restraint grit their teeth and button their lips? Or should the "silent" majority realise that Britain is a diverse place, and that diversity means difference?
Despite all this, my students had a great time. Many were breaking new ground. Some admitted they had never been to a theatre before. They loved the play and produced some perceptive reviews. They will go again, they said; although next time they might wish for a different audience. The sad truth is the same could be said for the National's regular matinee-goers.