I am sure that there is a lot to improve regarding that floor! What could be done for example is remove that carpet and the floor boarding and replace it buy a good isolating material and a good vinyl or laminat flooring!
Good luck for your project :)
A similar story occured a few years ago in my neighbourhood. You should contact the builder and put a little pressure on him so that the he really tries to solve the problems. Otherwise this could become a never ending story.
That's a difficult question, bruce. If you have a written document by the flooring expert you can show it to the NHBC. If they really consider it, that's another question. But that's the thing you can do to prevent further troubles.