Jardy, I know it sounds bad, and I nearly put all the details in to the reply early bird, but then I felt like I was being defensive, so I just put the bald facts down. But I will explain.
The hedge situation was a bit complicated - we wanted to build up to our boundary, which meant building where there had previously been a garage with a side-access pathalongside it. Next to this was the edge of her garden, which was planted with a tall hedge. Hope you can visualise.
Before drawing up our plans, we asked her if she'd be willing to remove her hedge (as there would be a wall in its place which would offer the same/more level of privacy than the hedge), and also because this would make the building work much easier to do. She was very relaxed about it, said no problem, she wasn't that bothered about how her garden looked etc.
To be honest, I felt a bit awkward about it, and so made my DH also go to ask her about it a few months later, before the architect got going with the plans. Her response was the same i.e. no problem, so we went ahead with the plans, engineers' drawings and so on.
The day before the builders were due to arrive I went to warn her that the work was due to start the next day. I also said, "Are you still ok about the hedge?" assuming all would be well based on the previous conversations. She said she didn't know what I was talking about and denied having given us permission. Not knowing what to do, I suggested that maybe she would like to discuss it with the builders, hoping once they explained what was due to happen, she'd be ok with it.
The builders arrived the next day, I told them she wanted to talk about the hedge, they said they didn't want to wait til she got home from work and they were planning to take the hedge down straight away (I had not known they were planning this).
I know that what I should have done is say "DO NOT TOUCH THE HEDGE" very firmly, but I didn't. I felt under pressure. I also had to leave the house to take my son to school (DH was working away at the time) and so I kind of didn't say anything more and just left. The builders, of course, tore the hedge down. I have to accept responsibility for that, which is why I went round to her house at the earliest I could (about 2 hours after she had arrived home from work) and apologised/grovelled and said we would replant the hedge once the work had been finished. She didn't approach us, it was always us approaching her.
The grass was damaged because the builders had open access to her garden (no hedge there!) and drove their tractor/machine thing on it. I also went to apologise about that and promise new turf (this was day 2 of the build). I was not responsible for that damage, and indeed DH and I were really annoyed with the builders over it and told them, but you know how builders can be. They don't really care - just want to get the job done and dusted. I was unable to apologise this time personally as this was the point that she stopped talking to me/answering the door, and so I had to relay a message through her (teenage) daughter, and put it in a note.
I feel as if accepted that the hedge thing was my fault, but that we did our best to make it up to her. She never objected to our planning application, yet seemed surprised that we were building a two-storey extension (I discussed this during the hedge-apology when she was still willing to talk). Our plans were all based on her being ok with the hedge removal (although having seen it torn down I can totally understand her upset, as it looked like a bomb had hit the place).
But what can we do? We can't turn back time - the damage was done, we accepted responsibility for it made genuine offers to make amends, all of which have been thrown back in our faces (I know I sound petulant). Added to that has been what feels like a campaign to 'get revenge' (constant dirty looks, strange behaviour with her bins, letting her grass become very overgrown next to our driveway - petty things really, but they build up a horrible tension). This is why I posted my original question, because I am so confused about her motivation. Is it revenge or is it a method of coping that is shaped by (possible) mental health problems.
Having written this, I feel that maybe the possible mental health issues shouldn't even come into it. I feel presumptious thinking about them, when I don't know anything really about her.