It is very common for builders to use the cheapest materials they can find and in your case it sounds like the carpet is not fit for purpose. Although, it has worn out and this is obviously the case I would recommend getting an expert opinion. The easiest way to do this would be to get the opinion of a reputable carpet fitter in writing with a quotation to replace the carpet in the areas recommended. I imagine this will be the high traffic areas such as the stairs, hallways and living areas.
As your carpets were included with the house you are not covered by the Sale of Goods Act. If they were you could give David Mclean two weeks to replace the carpets, before getting another supplier to replace them and billing the original supplier. Trading Standards would help you out for a very small fee and they would have no problems getting your money back from a large established company such as David Mclean.
However, as this does not apply in your case I would, assuming the cost is less than ?5000, use the Small Claims Court. This can now be done online very easily. Nik Clayton over at
New Home Blogs has used this and
others have used it successfully to resolve snagging issues.
I hope this helps
Let us know how you get on
Tony