Sorry, I did watch it but Answerbank went down before I could reply.
It seemed very odd. Does it happen every time now? I assume it's for the benefit of those who are visually impaired and want to know what images are being shown. It does seem to get in the way though for everyone else.
I assumed there is a button on the remote for it and/or it's an optional addition service, but from sir.prize's post and the youtube link it seems it was on like this.
Maybe sir.prize can clarify
factor - I watched it on Channel 4's 4oD during the evening. I don't know if the TV version is the same. The Audio Description only occurred about five times during the whole programme. Not much help to anyone really.