In reply to victorclimber: Wrong wrong wrong, I have organised literally hundreds of charity abseils over the last 25 years.
I strongly believe they should be run from structures, towerblocks,bridges,monuments,viaducts,lighthouses. there are all sorts of interesting (and scary) artificial structures to run charity abs.
Absolutely no need to run one on a natural crag. Remember, the members of the public will be turning up with all sorts of footwear, some of them will be very nervous and scrape their way down, some wil want to do "commando" jumps, whatever, the impact on the grit will be massive, compared to the careful footwork of climbers in modern footwear.
Complain to the NT. Brimham is a special and finite resource.