Eldon use the Anchor near Tideswell quite a lot these days. It has a big room at the back that can seat 45-50. Food and beer are great, and some of the staff are alright too ;-)
Then for something a big bigger there's the Mechanics in Eyam where DEF was held, when we fit in ~100 for a lecture type setup, and there's a bar upstairs. Big kitchen to sort your own food in.