NEC3 ECSS Defect Rectification when the Sub-Contractor doesn't believe it is their responsibility.

We have notified our tarmac surfacing sub-contractor, who were employed under an NEC3 ECSS 2005 with 2013 Amendments incorporated, of a defect in their works that needs to be rectified. Multiple discussions later and they still don't accept full responsibility for the defect and thus are requesting a 50/50 split of "costs" (yet to be assessed). They are well past the defect correction period of 2 weeks from notification so I could employ another sub-contractor to carry out the works, however, I would expect costs to far exceed their remaining retention monies and trying to claim any cost back from  them after the fact would likely be exceedingly difficult. Do I have any other options other than negotiation and, failing that, more formal proceedings?