What a neighbour of ours did when the main trunk of her trees was leaning over the fence line boundary was to have the new fence erected but to get the fencing contractors to cut out narrow "U" shapes in the panels at the relevant points is to allow the tree trunks to lean over the boundary line unrestricted. You can legally of course, remove any branches from his trees which overhang your boundary but by law you are supposed to offer them back to him. irritations like this can cause real problems between neighbours so it's best to liaise with him and try to come to an amicable agreement, especially if his overhanging branches are spoiling your enjoyment of your garden.