As far as day time availability goes, there's plenty - we have singing lessons in Every Saturday and Sunday daytime but other than that it's mostly empty.
As far as evenings go we have 12th & 13th and 17thth, 18th, 19th, 20th I'm guessing you probably don't want to get any closer to Christmas than that.
The cost would be dependent on whether you needed any of the sound equipment or not, if you don't I can probably arrange for you get the room for free if you do need sound equipment then it would be around £50 per night
We need to plan this fast if we are serious about putting up something before xmas, remember we need enough time to promote it ( 2 weeks)
thanks
Ed