Boundary issues

Questions:

• Can I cut some branches from my neighbours hedge if they are overhanging into my garden?
• Can I put barbed/razor wire/broken glass on the top of my fence to stop people getting in?
• I am in a dispute with my neighbour over the boundary of our land, can the police help?
• I want to build a greenhouse / conservatory, do I need planning permission?
• My neighbour has erected a fence and it has been built on our land, not on the boundary, can I take the fence down?