Like Eyebrow I don't understand how you think you would be able to adapt his existing guttering to give you more space for an extension.
The long and short of it is that it shouldn't have happened. However this should be been raised at the time when it was being built. The LR title plan shows what are known as General Boundaries. However if you have taken a straight line between existing features such as walls or fences are these pieces of guttering project into that space on your side then you are probably right. However getting him to move it now without involving expensive legal resources is probably impossible. Many homeowners have spent silly sums of money on boundary disputes of a few inches with their neighbours.
The Local Authority will not want to know � it is nothing to do with them.
I suggest you try confronting him with the situation and ask him to move it. But it is hard to see what he can do about it if the water flow on the roof is towards you.
That then means you will have to stop your build short of the guttering to leave a gap. Because you are building so close to the boundary, you should ensure you use the Party Wall Act during construction. See below (download the booklet) about your obligations and his rights. Of course he should have also used this Act during his build � but its too late now to do anything about that.
http://www.communities.gov.uk/publications/pla nningandbuilding/partywall