As I understand it from musicians I know, if you have a valuable instrument that's over the size limit for hand baggage (eg a cello) you buy it a seat (I understand seats bought for this purpose are exempt from APD). From BA's point of view, I can understand why they want to make the change, as someone has commented, why should musicians be treated any differently to other passengers? From the ISM's point of view though, I can see why they are protesting.