Neigbour disputes are a minefield, as i am sure you are aware.
From my experience, it's always best to go in 'soft', and have 'hard' as an option if being nice doesn;t get a result.
Call your neighbour round for an informal chat - always better on your territory, and explain the situation nice and calmly. The approach to use is that you are constrained by officialdom - all of us in this together, and thanks to their rules you can;t proceed with your border arrangements.
If you try it this way - both of you against the powers-that-be, it could work. Of course, if your neighbour becomes unreasoinable, and they may, at least you have tried, and you can tell them that you did want their co-operation, but you don;t have to have it, and you will proceed down the legal route, if that's the way they want to go.
If they insist on being awkward, then get the Planning Department involved, and let them take it all foward for you.
Hope things work out - good luck!