Beechhurst

Can any one confirm the clearance needed from a gas fire vent box which extends horizontally from the outer wall of our next door neighbours property. Whilst I have not found anything specific about the matter, most indications about other flue/venting clearances are that there should be a min. of 600 mms into “free space” or to the boundary between the properties – if correct neither is observed, even if makes the application from the wall exit point and not from the furthest extent of the vent box itself which is c200 mms in length – my concern is that gaseous emissions will continue to directly hit our brickwork and potentially be absorbed by same.