Have you contacted your local authority Building Control department about all this?
Ask the NHBC for sight of the certificate(s). If they can't or won't show you then it is unlikely that they exist and therefore unlikely that any checks have been carried out. Many will say 'How can this be???' but you would be very surprised at the amount of work on these sites that should be checked, but isn't.
I would bet that Persimmon's expert is an expert in nothing and so would be very wary of anything they say.
When a supplier (Baggeridge Brick) turns down a solution you have got to get worried. I had an incident with my favourite builder, Gladedale, whereby the brick supplier's agent slagged off his own company and its products because he was standing next to a Gladedale employee at the time. The problem with the bricks had absolutely nothing to do with the bricks themselves, it was the way they were used in the construction i.e Gladedale's poor workmanship. So this agent made a fool of his company and himself just to please his customer. Funnily enough the problem was no dpc or weep holes which allowed the bricks to be constantly saturated and spalled at the first sight of frost.
All sounds very fishy.
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