Who are you hiring the room off? If they do not have PLI Event organisers insuranmce for all the people coming in, then you need it. That is the premises responsibility normally and I would be surprised if they don't have that. I think it would cost you around £65
You need the musiscians side to cover yourself whilst playing incase you injure somebody.
The Village Hall who I hire from, has it's own PLI, so I don't have to do it. It is included in the cost of hiring the hall.