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I just wouldn't trust that the neighbour had Building Control approval if they haven't bothered to get a party wall agreement. Having seen some awful and sometimes dangerous loft conversions in the past, I don't really trust people to do the right thing.
At the very least, I'd ask building control if they inspected the property as it only takes an email. I'd also take photos of the existing movement, and see if any old photos exist.
If the property is rented then I'd be making sure that the owner has had a building control sign-off. You could call BC to check that this was done. You wouldn't get a loft conversion done yourself without doing things properly, would you.