Went down Cuthbert's last night with a thin person who said of Dallimores "Can't do the third squeeze"; it was also added that the cave is so tortuous that the original pushers used to leave spare helmets, Petzl Stops etc. beyond the squeezes (in a sort of kit store) - which they did in the dark with little kit on; the people who have been mentioned as Dallimores stalwarts are all so thin they look like they've emigrated from Ethiopia.
So presumably, Dallimores is neither hard nor a boring muddy hole; it's just plain impossible.
Remember, a cave is an underground void sufficiently large to allow the egress of a human whereas anything else is just a narrow fissure.