My neighbour's boundary is the wall of another building, which they grow plants up. The owner of the property has had most of it repointed, but the neighbours have stopped her, because the builders might damage her plants ( they were doing this before the plants started growing). Has the owner of the property a right of access to maintain her building. It was in a dangerous condition, as it was very old. The neighbours garden is a bit of a mess anyway and I cannot understand why they have stopped it. They also had to come on my property to repoint as well. The neighbour has also been round asking for compensation for a plant that has been broken! Great neighbours (NOT).
Divebuddy is correct. It is covered by the Access to Neighbouring Land Act 1992.
Google it, get the basic facts you need to back it up, then try having a quiet conversation with the neighbours. Obviously better to deal with it that way than resort to the courts for permission.