Google CJs at the Townhouse. They have two or three every month. £30 or £40 depending on afternoon or evening.
The prices you quote appear LOW in light of the other various gangbang parties that appeared to be rife last year before the newspaper expose at start of this year.
Have you been along to one perhaps you could give us a review as it appears VFM.
Googling the name you suggested give a website that purports to be in existence from 2005 to 2014 - can't say I've ever heard of CJ"s being mentioned before.
Is it a swingers group arrangement?
I note it states for any newbie -
"On your first visit you will be asked to bring along a form of ID. Without this you will not be allowed entry. This is for the security of all guests attending. Acceptable ID are passport, driving licence or utility bill"