INTK Theatre set murder mystery

There is a theatre show but somebody is killing the performers, however he is also making them carry on with the show. I believe he was announcing things over the tannoy. I think someone is killed under the stage. I also think it ends up with a character climbing up a big ladder above the theatre. I also vaguely remember a scene with a fire hose? It was like a murder mystery. Does this sound familiar to anyone? I watched it years ago on TV. Any help please? This has been bugging me for ages.