BM is spot on as always..........just wanted to add that NHBC and Zurich are not at all good at helping purchasers (putting it mildly).
First recourse should be the builder, then Zurich. If the builder is at all concerned over his Zurich rating for insurance purposes, then it's down to him.
NHBC, Zurich etc are only interested in selling the insurance these days.
Make a full list of the problems, keep a copy, and send it to Zurich.
I don't like the sound of this flat with all its problems. I would seriously consider employing a surveyor (ask at your local friendly estate agents). He would be able to back you up with professional advice............because when Zurich have rattled the builders.........I promise you'll be seriously fobbed off..and you'll need someone on your side.