Christian McArdle wrote:
(b) where the work is to a party structure or external wall,
subject to carrying any relevant flues and chimney stacks up to such
a height and in such materials as may be agreed between the building
owner and the adjoining owner concerned or, in the event of dispute,
determined in accordance with section 10

Indeed. You don't need their permission. If they attempt to withhold
it, it will be granted by a court instead. However, if their flues
intrude across a straight dividing line between your houses, it
doesn't give you a right to pinch their property and close off their
chimney!

Christian.

This is costing money is it woth it?

(b) each party shall appoint a surveyor and the two surveyors so appointed
shall forthwith select a third surveyor (all of whom are in this section
referred to as "the three surveyors").
--
Sir Benjamin Middlethwaite