Hello, I am a solicitor with 20 years experience. I will try to help you with this.
On the assumption that you are right about the boundary and that your neighbour accepts your assertions about the boundary I would suggest the following. You can put up a fence at the boundary point as long as it does not intrude onto your neighbour's land.
As far as reducing the size of the pond is concerned this is much less clear. You bought the land with the pond present when you purchased it. Do you know when it expanded in size? You do not know why it has done os Even if you can pin the reposionsibilit fro it on yur neighbour rather than it being the action of nature.
You will probably be stuck with it under caveat emptor.