The real answer is to keep your plants in check so that they don't go over onto the neighbour's side. Sad thing is that you replaced the fence at your expense and new neighbours don't appreciate that. The only way of finding out who is responsible for the upkeep of the boundary structure is to check your deeds. It will be stated if you have sole responsibility, but do not assume that no reference means no responsibility.

ps Montana will come back as long as the roots haven't been damaged - it just won't have the amount of top growth.

You said that 'you fear that it isn't your boundary anyway', which leads me to believe that the fence is on your nieghbours land. If this is the case then they have a legal right to do whatever they want with the fence and anything that grows on their land. Technically, if you erected a fence on someones land and grew something it is trespass and they could force you to remove it.

The only way to solve this legally would be to do as you said and erect a fence on your side of the boundary.

I had a similar problem with my nieghbour, she took it upon herself to trim the height of my berberis so she could talk to my neighbour on the other side of me. I wouldn't mind so much but she just cut a dip in the middle so that she could see through it and it looked awful. I erected some trellis behind it to the height that I want it to grow making it impossible for her to cut the top below the trellis from her side of the fence.

What a nerve Myron! I bet you were mad.

Dollis is what you are asking how to transfer the hydrangea and clematis from the old fence to the proposed new one?

A visit to the Council Offices Land Registry Dept will solve
the problem of the line of the boundary.