Unless it is powdery, I wouldn't be too concerned at this time.
It will need to get much worse for LABC to get involved and even then they are in my opinion not the bets warranty provider.
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weak mortar Bit confused as to why there are small stones in th mortar, which should be building sand only.
I can only suggest they have used sharp sand which they should not have done. That said, the bricklayers would have
kicked off big time if that was the case.
As for the developer who does this as a side line to his main career/income, smacks at a lack of supervision and you may have ended up with a real defective new home.
My advice is:
1) Notify LABC now to get it on their system
2) Get your home professional inspected and snagged as soon as possible.
3) Write to your developer and tell him in no uncertain terms that he will end up in court idf he contnues to be so dismissive for your concerns!