This thread discusses the Content article:
Persimmon Homes Reneged
It is outrageous that this has taken so long and the fact that Persimmon Homes promised to do extra work as a gesture of goodwill which they have failed to do is completely unacceptable. I am not a legal expert, but I would say that you had a good case, especially if you have written evidence, if you went down the legal route to get them to honour their promise. The most economical way to resolve this would be to use the small claim court, but talk to your solicitor first. This can be done online.
Moneyclaim Online
I hope this helps and good luck
Kind regards,
Tony