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I'd leave it if i were you.
You're losing nothing here other than potentially any friendship between you and your neighbour.
If you build a dormer in the future you can join onto theirs directly and both of you will enjoy the benefit of that.
Provided their work is done to a high standard (the step flashing suggests that their lead man is at least competent) you have lost nothing, and they have taken nothing from you. You will gain nothing from pursuing it.
Any gap between is a stupid way to construct a dormer.
(edit: the drawings show the neighbour building up to the centre line of the party wall, which is generally assumed to be the centre line of the chimney stack; the 300mm is the distance between your edge of the stack and the centre line of the stack, they appear to have built as per the drawings)
I would consider very seriously if this is a road you want to go down.
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Dugtheslug
Thank you all for your scrutiny. As @Cupcakes has pointed out to me privately, the drawings are sloppy, but under scrutiny; I don't think they are ambiguous on this issue.
I think I will broach the conversation suggesting contractor error. We do have a party wall surveyor appointed prior who might be able to help too. So I'll contact him.
@6pt Yeah, I've contacted Lewisham to ask about enforcement procedure. Because the website says they just won't for permitted development issues. Seems pointless to get a certificate from them.