Hi
Your solicitor will tell them what will happen if they do any future damage - injunction etc, costs to be borne by them. Police will take notice of court injunctions. Yes it will cost you a bit as well, (solicitor will tell you how much) but WELL worth it. What price peace of mind! You are the ones in a strong position here. Their argument should be with the people who sold to them. That will teach the original neighbours not to fuck with you!
Evidence - Take photos of the new fence and any previous damage. Write down everything that has been said and done by everyone, so far, whilst you can remember it, and if there is any more trouble keep a diary, with dates, recording it all by photos and security camera and your own notes. If you ever involve the police again, make sure you get a case number and the officer's name.
(I bought a Dictaphone thingy and carried it in my pocket switched on every time I went near my fence and thought there might be a confrontation, for 6 months, I never had to use it though, I was worrying unnecessarily, but it made me feel prepared, it wouldn't be evidence for court, but would be persuasive evidence for police.. And I kept my camera in the kitchen and used to run with it every time I heard the neighbour start up his chainsaw.....I would have had a LOT more peace of mind if I'd invested in a security camera and more importantly, if I'd got my solicitor to write a strong letter, I could have saved myself months and months of angst.).
A solicitor's letter is by far the best thing and should put it all to bed.
Take heart, I know it is very stressful, but right and the law is on your side.