Hi Pete,
If everyone on the list turns up, it will definitely be tight for space. However, there are invariably no-shows for these things, so we can only really know on the day.
My best advice would be to bring your gear, but if there is no room, leave it in the car. There may well be some leaving halfway through and freeing up space.
If you're nervous about leaving stuff in your car, I'd just bring the bare minimum eg one bass
Cheers