Have you thought that if this woman is living at home with her elderly mother that they may not have much money to have done a fence immediately?
They may have spoken to someone who said he couldn't put up a fence without damaging your shrubs, our old neighbour had to walk on our border and theirs to replace their fence and despite being careful they did damage our plants. Just because they said something that didn't agree with what you were told doesn't make them malicious or liars.
You say you contacted the dog warden 'the next day' and you are already going on about claiming damages through the courts and collecting evidence. What are you going to do DNA their dog just to make sure it was its poo?
You have listened to others opinions about them and condemned them without giving them much of a chance from the sounds of it. Do you know word for word what they 'said' about you or have you had chinese whispers from other people??
Perhaps they have a right to be upset as you appear to have been confrontational and agressive over an incident that could have been solved with the cup of tea and discussion in the first place rather than dog wardens demanding they repair things in 7 days which is threatening to two women alone (and indirectly came from you). If they didn't repair in 7 days what was he going to do??? Take away their pet and destroy it???
Perhaps you need to take a step back and think that perhaps other neighbours are bad mouthing these women to you just as they supposedly are doing to you.
Have you thought that all this may be making the old lady ill and may contribute to her death??? It certainly isn't going to be make her last few months/years whatever stressfree and unhappy.
I think you may need to take a step back and calm down you do appear to have been very confrontational over the first problem you have had with them.
And just incase you are interested our previous neighbous had 7 cats who used our garden as a latrine (nothing you can do about cats they are outside the law) and the current ones scream, shout, swear, rant and rave nearly 24/7 and their kids call my DS's 6 year old bestfriend a 'paki'.
This has ended up a lot longer than I expected - sorry (-: