What should happen is the NHBC take over the problem and appoint their own contractors to carryout the necessary works. This make take a while, as the NHBC prefer the housebuilder to fulfil their obligations under the warranty.
When the NHBC used to give detailed figures in their Annual report (they don't anymore) nearly 30% of the total cost of claims paid out each year was to fix defects in the first 2 years when the housebuilder should have done.
I would expect Dawn Homes will have their NHBC builder registration cancelled.