Yes you may, when I bulit my extension brickie started doing dividing wall between us and neighbour and it meant slates would be over hanging by about a foot,a family friend in the trade saw this and advised he restart and us a parapit wall so no slates would be overhanging, brickie had to take down some of the blocks already layed. if I had not done that if I had a disagrrement with neighbours who are great by the way but Murphy's law and all that, they could have made me take the slates which over hang their land. It was also suggested at the time for me actually to buy that foot of land of my neighbour, I dont think the land part would have been expensive but more the legal side. So if this is already done then the problem is really with him and not you if nothing is in writing to state that he had got permission to overhang your land. If you sold your house the new owners could make him take it down I think. he would have issues if he was to sell his house. so I think he needs to sort this out with you.