Pancetta Mold. How to tell whether it's safe to eat?
On another couple of sites (Ruhlman for starters) I found the answer to this. It appears that mould is expected sometimes in the Pancetta making process and as a general rule of thumb as long as it isn't too green and furry then it should be OK. I've followed Ruhlman's advice and dabbed the spots of mould with a vinegar soaked cloth and it wiped straight off and looks fine.

My meat was coming up to the end of it's hanging time anyway so rather than worry about it further I've refrigerated it. 

I hope this helps anyone that stumbles across this with the same issue.