Being a landlord qualifies you to make judgements on a boiler thats not even fitted does it?
Why do landlords always think they know best?
Actually the answer to that is YES - not because I'm a landlord, but because I know the properties. The house and the flat were built at the same time, using the same model boiler, and almost certainly done by the same plumber (and sparky, I've issues with him as well).
So based on what I know, I am quite confident about how the original boiler would have been fitted - and fairly certain that anything NCS as pointed out in the BG service report would have been very minor at worst, certainly nothing to justify a new boiler.
Edit: the service report didn't say what was considered NCS.