Grout keeps cracking on new tile floor installation

[Chalker]

Looks like 12mm ply to me. Bet it's fastened down with builders favourite. Clout nails. Then the cheapest powder adhesive available. Get him back and tell him you are going to get an expert opinion and have it relayed at his cost ( upfront)
i wouldn't try the courts myself as it will take ages and if he has nothing and is a limited company your chances are slim.
Try and appeal to his better judgement , if he has any.

 

[White Room]

What concerns me is the thickness bed of the adhesive, I can't believe they would have laid it that thick all over.....

 

[cam_low]

I think its dot n dab then they have filled the side.