In reply to nniff:
Thanks all - At least it's nice to know it's not just me.
nniff - I do pretty much the same as you in terms of just moving to a different route, but I'm finding as I try to move onto harder climbs (which is around 6c for me) that heel-hooks become a more and more required technique so would love to fix it.
I think my best bet is to try using them on easier routes where I wouldn't normally to see if that builds resilience in the muscles concerned.