Your neighbour has no right to grow trees or anything else on your roof, so I suggest you go round to see him, and tell him that it is necessary to replace your garage roof and request that he cuts the trees back. If he is so unapproachable, then write him a letter. If that doesn't work, then get your solicitor to write to him, but don't get into any lengthy litigation, it can be too costly and it would probably be cheaper to get the branches cut away yourself, which you are quite entitled to do. Just give him the branches back.