Martin Collett, a rider who did this route fast on a road bike in 2011, has constructed accurate GPX files of the route, which includes detail of the forest tracks as well as the roads.
I'm told they work with Viewranger, a useful mapping app.
Open this zipped file and you can either download four separate GPX files (see Martin's note below) or a single file if your GPS can handle a large one..
Martin also says that it also helped to read the route guide notes first, especially for the forest sections, just to get a sense of orientation.
My places map and offline access
In addition, Peter Kidd has made these files into a published "my places" map. He did this in order to be able to download the route onto his tablet and use even without being continuously online. Here is what he found:
"Although one cannot get access to 'My places" maps without an internet connection, I realised that if you load such a map onto a mobile device while one does have an internet connection (e.g before setting off from home), and then switch to Flight/Aeroplane mode, the loaded map and route remain accessible. So, for someone like me, who has a Google Nexus 7 but no smartphone, it's possible to carry your electronic route map as well as the printed-out paper instructions."