Hi, I'm the other Walker Gp Home owner on here!
You are in a difficult situation, as your problem falls within the responsibility of various people and is a grey area and can be argued by all responsible parties to try and get out of doing anything.
Think what I would do is write to all parties involved and gauge their response, so this would include Walker Group, NHBC, Contractors that fitted the windows and Company that guarantee the windows.
Personally I think responsibility is on the contractors that fitted them, I assumed these incorrect fitted items cannot not be seen without removing windows and checking? Saying that Walker Group hired these contractors and if they still work for Walker Group they may be able to apply some pressure to sort it!
It might be worth a shout with Company that guarantees the windows, write and say you want to claims as windows have not lasted, they will send someone to check that they are indeed rotten (don't say why they are, play ignorant) but may not notice why this has happened!
Failing all the above gets you a free resolution to your rotten windows, I know a good, reliable & competitive in price terms glazier. Message me if it gets to this stage and don't know a good one yourself. I am just a stones throw away in the new Meadowcroft estate.
Finally just thought, is there no way of claiming for this on your house insurance?
Good luck
Ant