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I should have been clearer on the Building Control aspect: the previous owner did not seek building control approval of his works last year as he argued he was just a property owner choosing to upgrade an existing situation. He felt he wasn't doing any works because someone was requiring him to - the fireproofing and soundproofing were just done on his own initiative, to "do the right thing". Only the rewiring needed approval, which was done via self-certification by the electrician. He didn't want to have his builder's work assessed against contemporary building regulations - too much hassle, I suppose, and if Building Control had required approval and certification, he would have given up and sought to sell the flats as they stood, i.e. with Planning approval but no Building Control approval at any point.
What I meant was that I had Building Control round to assess the situation last month, and they said it appeared that the fire protection works were well done and would satisfy them, but they couldn't formally sign off against 1992 regulations without lots more detail from the builder (now moved) or physcial investigation of the fabric - taking up the upstairs flatholder's floorboards, for example.
My mortgage company and surveyor were satisfied with the quality of the flat to allow me to buy, and no-one, including myself, sought to investigate Building Control approval. I've offered an indemnity policy to my buyer but her solicitor has been saying she needs Regularisation, because although the insurance protects her against a Building Control action, it does nothing about the fact that the flat does not any certification - like hundreds of thousands of other flats in the UK.