OK, I have watched the youtube video, and I remember the prince saying about taking Dr Watson hostage, but for some reason, this event did not trigger what happens in the video. The situation I am in is that the Prince is refusing to speak to me, and Dr Watson and Holmes can wander round freely but not access any more of the theatre. When I approach the Prince, he simply says something to send them away and I can't initiate any more conversation. So I can only think this is some kind of glitch in the game, I will have to try and reload from the last auto save but I'm not sure how far back that is
oh dear, well I'll try and go back to an earlier stage and see if I can make the right thing happen ... thanks for your help eveyone so far, I will let you know if I get past this!