Common new build problem grit small stones etc more likely up against stopcock taps etc. I always use to blow the main out through a window before connecting.
To late now though and its 18 months on so past the 12 month defect period. As already stated pay me for last job before i go back. Obviously never go back and block his numbers
I always disconnect the main and flush through after a new connection. The guys who connect it in the road just don't care and always get grit/mud etc down the pipe.