Rob, I rode the route just before Xmas, I think it was about then and at a point somewhere after Sproatley, the route goes across country where not even a road or track exists, just a grass field and hedge, I just kept following the main road and eventually rejoined a good distance further on.
I'll look for my download then I can be more precise as to where it deviates, plus there is that odd spur at Sproatley where the route doubles back on itself, even though I had noticed it earlier, I forgot about it and just carried on and made a bit of a loop to rejoin further on.
Unless anything unexpected turns up, I should be there with my fancy Garmin dodah thingy mejig with the route slightly different and known to be on tarmac.
I've just checked your route Rob, and its just after Hilston when you rejoin the B1242, you come to a very sharp right hand bend, your route carries straight on into the fields, which then becomes a dirt track, then joins a road eventually. I carried on along the B1242, took the first left "Back lane" and then onto Humbleton road and rejoined, it was also the same distance so it was not a shortcut.
If you zoom in using sattelite view, its clear to see the error.