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have you actually read what the indemnity insurance covers you for?
you are conflating the i) purpose of the policy (which is to cover your financial losses in the event the council enforces against you for not seeking building control approval for notifiable works previoulsy undertaken at the house) against ii) the fact you havent told your neighbours you are undertaking work on the party wall in respect of the chimney and iii) the building control bloke coming round and saying the work is good, safe and complies with the regulations and signs a bit of paper to say as such.
the chimney was actually removed from the groundfloor up previously, they only removed a few courses of bricks to fit the ensuite in better. no party wall was arranged when that was done previously, so i'm not sure whether the indemnity insurance i have with the purchase of the place will cover the plod from the council who comes round or not. we shall see.