Richard, I stand corrected. However this appears to be an exception for musical performances to the general principle that (so far as I am aware, and I am basing this on advice on photography forums) it is lawful to take photographs and videos in a public place.

The Act is presumably intended to cover formal performances rather than the more casual sessions and singarounds which we are familiar with. Outside concert situations it is both impossible to enforce and often impossible to comply with - will the BBC have obtained consent from all the football fans whose singing it will broadcast on "Match of the Day"? More seriously, at sessions I go to it is common practice to record and video (usually on phones rather than with the full kit) but with numbers of musicians joining and leaving the session over time it would be impractical and disruptive to seek their individual consent. But in that context I have never heard anyone object or suggest that it is unacceptable.

Even if recording is unlawful, what steps can you take to stop it? The Act refers to the performer's rights being infringed, so presumably this is a civil matter in which the police would not get involved (although some of the responses (although some of the actions suggested in this thread could lead to a breach of the peace). Presumably the performer would have to take legal action for infringement after the event. Perhaps there's a sideline for you seeking instant injunctions on behalf of disgruntled musicians - I know a judge :)