I'm back with an update although not a particularly happy one on the progress with my neighbour.
Everything was going well until recently. The neighbour said a while ago that he was having a damp problem along the wall that currently forms the boundary so I agreed that he could remove the soil from along the wall and put a *small* line of gravel along it to improve the drainage. Whilst digging this channel, the neighbour say he found an old fence post(s) that must've formed the original boundary line and so has decided to create his border out to this level. The result looks like this:
I went over during the day yesterday to have a look and had a dig to find the fence post concrete he'd mentioned - it was a lump of concrete that also had some mortar/render on the side of it which struck me as strange, but with a flat section down one side that might've been a fencepost. I'd have expected that to be in the middle though? I had a dig further along the line and didn't find any others along there - it would seem that unfortunately as part of his building work the others were removed.
My complaints about the size of the border have been met with flat refusal to do anything about it as "it follows the original boundary", even though this doesn't seem to correspond with my previous photo that shows the retaining wall he discovered early this year.
When a family member knocked on his door to complain about the soil and rubble that had been thrown out across the lawn and was preventing the grass from being mowed he stormed across into my garden and was thoroughly obnoxious and abusive (bordering on funnily so). He's threatening to "go to law" about the issue now.
I think that current his motivation is to be able to gain access down the side of his property to "put up a pair of steps and clean my gutters and fascia", as he puts it. He wanted to put a gate in his fence where the new extension ends so that he had access (although he said I could lock it from my side if I wanted), rather than having to walk a long way around to come in through my front gate (which fronts on to a different road). I've flatly refused this now, so he's said he'll either climb over the fence or take it down. I'm aware that I can't refuse him access to maintain his guttering/fascias but I think he's supposed to give notice first?
The other 'motivation' is possibly that I've reported a new dormer window he's built above the wall pictured to the local planning officer. It looks directly into the whole my garden and has clear glass that opens. I asked if he'd consider using obscure glass and he said "No, I don't have to". I know he didn't apply for planning permission and after some reading of the permitted development rules it would appear that it doesn't meet those either as it's a side-facing window and as such should obscure glass and not open.
Overall, I think it could be worse but he's absolutely impossible to talk to now and just flies into a rage whilst refusing to listen.