Markcdo - Try the Army Mountaineering Association; they reguarly put out requests for ML's etc for Mountaineering Trips / Climbing and there may be some people who also have a MTBL Qual who can assist.
Sorry I cant help anymore!
Any to the others, as stated yes you may have to pay £300 to go on the trip but generally you'll get your rank pay X 5 days (for example!), accomodation, food etc. Still a bargain in my book! Good luck