Ok, this is fubar...
I have my edited terrain tile, and switched it with one of the existing ones. I then created a set of the plb files (for where the terrain overlaps another tile of a different type) and that worked.
HOWEVER, I decided I didn't like the look of the overlap, so decided to change it.
The problem is now I can't get rid of it!!!
I've deleted the overlap texture entirely, (it DOES revert to the original texture that came with AH, until I try to put my NEW plb files in, then it reverts back to my old ones!) but no matter what I try to do to change it (modify what I made, make an entirely NEW one) I'm still getting the EXACT SAME blending between my new terrain and other tiles! I've tried completely deleting ALL the terrain variants from the texsrc folder, and copying the whole thing back in and it STILL doesn't work.
And I know it's the same one because the new overlap texture is COMPLETELY different.