I am not sure how much it would cost, we have just had a tree removed from our back garden and that cost £500. The line is exclusive to his property, I cannot say how long it has been there as we only moved into our house last October. I do not want to remove the tree and even if we cut branches it will still be a problem as BT (or Openreach) has routed it from the telegraph pole straight through the tree as it is the shortest distance.
I very much doubt he will pay anything towards it as he told the guy who worked in our back garden that any cost would be payable by us. Quite frankly I would be happy to do nothing as he has not even mentioned it to us except in passing to my husband, all he said was there was a problem with his telephone line which was why my husband suggested to the gardening guy to knock at theirs to see if he wanted any work done.