It's not really a showcase for bands. The format is more a mix of players from a number of local bands, putting together ad hoc bands for the occasion, no rehearsals, just a set list (which more often than not, gets forgotten on the day) announced a few days before hand.
Happy to let you know when the next one is though. It's a good place to meet the local musicians, introduce yourself and put yourself forward.
I know the next one will probably be July, when we will be raising money once again, for Great Ormond St Hospital.