We saw a preview of this without realising, and may have misjudged it. We left at half-time, knowing how it would end having seen the film (though A in fact hadn't and we had to explain the denouement to her) and being underwhelmed by a script which seemed to add little to the film (they kept Vangelis's music) and by pedestrian performances.
I now think the performances would have improved by opening night, and a certain lack in that department may be excused by the fact that in rehearsal they'd obviously been concentrating a lot on choreographing a physically complex and demanding show. The reviews I've seen have been generous, likening it to a West End musical (there are G&S songs and a 21-strong ensemble -- so maybe Hampstead's hoping for a lucrative transfer and a West End run) (just after writing this I discovered they are: saw a full-page ad for the transfer to the Gielgud).
A figure-of-eight running track goes around the whole space, with seats around all four sides of the auditorium, the track running behind some of the seats, in front of others (Hampstead really is a most flexible space given that at first glance it looks like an entirely conventional proscenium theatre). There's a central playing area. The cast ran (really ran, and scarcely puffing at the end of it) around the track as well as indulging in group callisthenics in the central area. All choreographed with great precision.
I found it a bit dull and predictable, the physical side notwithstanding, but others clearly didn't. One of the advantages of this kind of staging is that you can watch the audience. Towards the end of part 1 we have the champagne hurdling scene (which is also of course where the smoking comes in, our insouciant aristocrat happily lighting up at every opportunity), and I saw one woman away to my right literally agape, quite transfixed, as she awaited the outcome.
Overall, it made one ask again what the point is of reimagining in a new medium something which worked supremely well in another.