Oh Dear me. I had a similar problem many years ago. I had a very narrow strip of ground by the side of the bungalow that I owned at that time. It had a few small shrubs, and one year, during the spring/summer, several very pretty wild flowers ie: ox eye daisies, poppies etc grew amongst them. As these flowers attracted the bees and butterflies, I let them thrive. I came home one day from work to find that everything, including my lovely little shrubs had been completely hacked to the ground. I could not believe my eyes. I visited my next door neighbour to see if she had seen the culprit.
Yes, and she knew who it was. It was her gardener, and SHE had instructed him to do it as she did not want any weeds getting into her garden even though there was a fence between our properties !!!!!!
To make matters even worse, she took great delight in telling me this. She was positively gleeful and laughed in my face.
She wasn't laughing though when she received a letter from my Solicitor.
Trespassing on private property, criminal damage etc, with my photographic evidence.
I did not want to take it any further at this time and told the Solicitor so.
His letter mentioned this, but he also added that should anything like this happen again, then it would be taken further.
I never had another problem.
A neighbour can only tackle trees, plants, weeds, that grow over or under the boundary fence. They can cut them back up to that boundary and no further. They cannot encroach on any land that is yours unless of course you permit them too.