A roof on a new home should not sag!
This is a major structural defect and is still covered by the ten-year warranty.
After 2 years, Bellway can refuse as the NHBC warranty takes over responsibility from years 3 to 10.
You need to make the NHBC aware in writing that the works they arranged have been unsuccessful and that they need to make a further detailed inspection to assess what is now required.
The house plans should be available from whichever body provided the Building Control.
This may have been the NHBC or the Local Authority.
In either case, you are entitled under the Freedom of Information Act to see them. Make a formal request using the procedure you can find online.
You can also make a
Subject Access Request to obtain copies of all information relating to your home. It would be worth doing this with both Bellway and the NHBC before you take any legal action.
It does seem like the NHBC are trying to hide something although what their motive is unclear.
The roof will need repairs and the faults will be found.
If as you say the NHBC contractors have tried to cover up works that needed doing then it could be disastrous for the NHBC's reputation.
Do you have legal expenses cover as part of your Home Insurance?
If so you may be able to get free legal advice.
I am not sure if this case would interest the 'No win No fee' brigade!