Dave,
I don't think this will get sorted until you sort it yourself........
If there's sandstone lying around the estate waiting to be installed, possibly some of it could make it's way over to your place. Don't do it of a night time though, just waltz right up in broad daylight and start loading it up........
If and when challenged you just explain that you're collecting your retaining wall.......
As for the dirt....................just push it back over to their side of the boundary line.
It's bad that you will have to pay for it and waste your own time but maybe it's time just to suck it in and move on? I know that's what I'd be doing.
ciao
Nor
( Thought I saw load of pineapples heading East the other day.........now I know who they were being delivered to.......
.......)