Following a comment from Mark Williamson on my last diary entry I was able to line up the NPE map with my ways and correct (hopefully!) some of my previous inaccuracies. I've also started playing tonight with JOSM, which highlighted a whole load of GPS data I didn't even know existed, so I was able to line up my ways with the GPS data in some instances. There's still a certain amount of interpolation where I have no GPS data (yet!) but it should be more accurate now.
Now if only I can stop myself spending so long doing this stuff... but it's so much FUN!