QUOTE(VH2 @ Feb 15 2012, 05:21 PM)
1. To perform it to an audience may require a fee to be paid to the composer or their estate. This is a matter of performing rights. Usually this is covered by the venue at which the piece is performed. It makes no difference whether you play from memory or from a score.
Ah, just noticed this. I don't think this is the entire picture.
Yes, there is the matter of performing rights - this is covered by PRS (Performing Rights Society) - regular performance venues must be registered, and must declare any copyright works performed. But the charge normally applies to the whole concert: if there is just one 60-second copyright piece in the concert, the charge applies to the whole concert. (What a marvellous way to encourage the performance of 20th-century music.)
But also, on top of that, you will normally have to pay for the licence to perform the music as well, if you are hiring the music. There will be a basic hire charge, but the hiring company will usually then add a substantial charge to allow you to perform that work in a public concert. (And they can dictate how you may use that music too: for instance, if you hire the parts for songs from West Side Story, and the Symphonic Dances from West Side Story, you are not allowed to perform the dances interspersed with the songs.)
It's all a bit of a confusing minefield.