There have been many problems with leaks now plastic plumbing is common place. whether the fitting was manufactured correctly or not, the house builder is liable under the NHBC warranty.

As Steve F says ensure everything is in writing and copied to the NHBC.
You need an extended warranty for the plumbing to cover you against any new leaks yet to become evident.

It may also be useful to know that the 'O' rings in the fittings have a limited useful life and can be broken down by Flux in the system from any soldered copper joints elsewhere.

Other problems arise when inserts are not fitted inside the pipe before jointing and if the pipes are not fully pushed home.