Hello, I am a solicitor with 20 years experience. I will try to answer this for you.
I fear that you do not have any legal recourse available to you. The neighbour can largely do what he likes on his land as long as he does not breach the law and there is no obvious breach here. There is no general right not to be overlooked. Is there anything on the deeds (checkable at the land registry) preventing this mound eg a restrictive covenant. Without something of that nature there will be no recourse for you.