I had awful trouble with my neighbour. When I returned from holiday, I found he had taken down the beautiful hedge between us, encroached about 8 feet in, and built an ugly breeze-block wall. Bearing in mind I was divorced, on my own, he probably thought, I would let it go. No way .... I took legal action. Needless to say, I won the case and was awarded costs. My legal Bill was nearly £5000, including Surveyors fees etc. Heaven knows how much his Bill was. He was ordered to demolish the wall, and take it back to the original boundary.
So RossPB, do take legal advice. I don't know whether Solicitors offer a free half-hour. Police rarely intervene in neighbour disputes.....
To think that London, or anywhere else for that matter, does not belong to any one demographic