In reply to UpTheDale: Awesome Walls in Stockport, and Kendal, are the only ones where you need a 50M. You can get by at the others with a 40M, even MCC, although there won't be much left on the floor after lowering off the big wall there! But as others have said, it's useful getting a 50M anyway, as it is the 3-4M at each end that gets beat up very quickly and it's useful to be able to chop that off and still have a useable length.
I've used Mammut Promos quite a bit, as they are usually pretty cheap, and they are fine, although as others have said they can vary from rope to rope and my latest one doesn't seem to be lasting as well.
Worth shopping round for one-off reductions on any rope - I recently got a 50M Edelrid Python at GoOutdoors, reduced from £104 to £52!! At that sort of price it's as cheap as a Promo and a better rope.
Cheers
Andy