A couple of video clips of the Good Friday passion play in Trafalgar Square have gone up on Youtube. The sound quality isn't brilliant, but you can hear most of it.
part 2 - gives you more of a sense of the size of the crowd.
Part 3 - plus helicopter! Good sound system though, seems to win out! Based on the clips, it sounds like the script went beyond a simple recounting of the gospel narrative.
Also a picture here, eye witness account at the Blog of Dan, news stories here and here, one of which estimates a crowd of about 20,000 people.