Any question answered...

my super expensive, super comfortable assos bib shorts have worn so thin at the back that when i'm on my bike you can clearly see an inch of arse crack through what now looks like thin, ladies tights material.
it's not pleasant for anyone behind me but the shorts are my favorite and i can't afford to replace them -in every other respect they are still in perfect working order.

any ideas how to fix them?

I've done this to many many pairs now. My solution is to wince and buy new ones :(