A couple of observations, with no definite conclusion. I am not a civil engineer, but I did replace my roof
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Where do you think the water is coming from?, e.g. is some part of the construction still "drying out"?
Are other properties suffering the same issues?
The membrane between the rafters and the tiles should be breathable, i.e. porous to moisture but not to rain. Examples: Tyvek, Daltex.
Your statement "...there were areas of no insulation at all in between the rafters, and that was the maximum ventilation..." is too loose to be correct.
There is a difference between a "warm roof/loft" and a "cold roof/loft". Different designs have the temperature gradient in different places (i.e. thermal insulation is in different places), and humidity barriers are placed so as to ensure that the dew point occurs in a safe place. You need to understand where the humidity and thermal barriers should be.
Do not assume that the building has been built correctly and to design. There are many many examples of on-site people cutting corners and/or just doing what they've done before. The former allowed an entire cul-de-sac in Sadly Broke to be built with only 25% of the specified cement in the mortar => mortar could be abraded by fingers, and (IIRC) Lovell had to buy all the properties back. The latter was the source of the "high alumina cement" building failures. You may have heard the phrase "Jerry built", referring to poor design/construction. That originated in the 20s/30s and referred to the various problems found in homes rapidly constructed after WW1.
I suggest that your first step should be to complain the builders and the NHBC - with other people if they are affected too. If there is a problem, do not expect the builders to respond easily. If that fails, then you will need to find a structural engineer/surveyor and solicitor that are prepared to take on the case. That may not be easy, since the building industry is a small community (think Masonic Halls).
If you can back out of a purchase, you might like to consider that. I would
not rely on a builder's statement that "it will be alright when finished", unless they state
why that will become the case
and you believe the cause and effect.