The NHBC accepted the assurances of Persimmon Homes engineer that the retaining walls had been built correctly by them. 

See my Persimmon Homes retaining walls

In fact if the NHBC had checked the two engineering drawings for my Persimmon Home they would have found they had been specified to have been built with engineering bricks. The NHBC still does not accept this is a breach of their Standards even when you send the evidence to its Chief Executive!!