Thought I'd better own up to having snapped about a third of the first crimp on Sunday
I watched a couple of beta videos online which showed people using it as a heel hold and thoughtlessly did likewise. It hasn't become any harder but if the hold is snapped off completely then it will very much change the nature of the problem. There's plenty of other heel placements should you need one.
I know this will seem like stating the bleedin' obvious to say "don't put your heel on small lipped crimps at Brimham" but I'm certainly not the first to do it so thought it best to try and point out the error to others.