Thanks. The issue was twofold, I'd inadvertently been creating the weight files as 2049x2049 because I was using the sat map png as the base, and the second was the 8 bit single channel one due to paint.net not saving in the correct format. I've sorted both those in Gimp and it's working fine, I'm pissed with myself because I should have spent a whole lot less time faffing on that and read the bloody GE console as the error was clearly stated in there. Lesson learned.
I'm now looking at the roads, and have had some initial success using street spline constructor on a relatively straight road but am now running into problems with some of the more twisty and convoluted roads, it either doesn't create the file complaining about angles too great, OR, it creates the road and there is a ruddy great 180 degree corkscrew part of the way down. I can't quite get my head around why GE changes the X and Z rotations of an object when I rotate purely around the Y axis, it flips from 0 to 180 or -180 and that's what's causing the issues, but I'm damned if I can figure out how.