Many thanks to everyone for the greetings and the comments on the lighting. I think we can conclude that it was a Picturehouse problem; I've raised it with them and asked for an explanation (and a refund). I'll let you know what transpires.
I watched it on live-stream at the Exeter Picture House. The lighting of the performance was terrible! I suspect that was a problem with the stream, rather than the actual lighting design, but it ruined the ballet. I'd be grateful to know if anyone else experienced the same, either at the ROH or in a cinema. Act 4 was particularly bad - you could just about make out Vadim's face and hands during the PDD but Rothbart just vanished into the murk. It was very disappointing.