My Mum bought a new home from a building company about 25 years ago. Her deeds say she is responsible for the fencing to the right (the posts are on her side as well). However, on the left hand side there are about ten fences which are have seen better days especially due to the recent winds. Her understanding is that the builder replaced hedges along that side with fencing, but the posts were placed on the neighbour's side. The neighbour says they have never owned that boundary although I'm sure they cut back the hedge which would in law I understand means that if you accept maintenance of a boundary then it's your responsibility. The builders have then replaced them which you can argue makes it their responsibility but they haven't passed this onto the owner (my Mum) and are now out of business.
She briefly spoke to her solicitor a while ago who suggested agreeing verbally on equal responsibility but the neighbours would not agree.