There’s no one route, it’s changed each year for various reasons. 2016 & 2017 were almost identical with just a couple of minor variations due to hydro-electric and bridge works. This (2018) year’s is slightly different again but it’s only been sent to those entered in the group start.
There’s a few threads on the Bearbones forums about the route.
Generally it’s easy to follow as there aren’t a huge number of tracks to choose from. The biggest problems I had were usually turning off roads onto the trails especially near villages as there could be a few options. Once on the right track then you’d have several hours of riding before having to make another choice.
Feel free to ask questions either on here, on the HT550 2018 thread on the BB forums or by PM.