I have been creating routes using plotaroute.com which exports to a .KML file, and I have no trouble with this process, and IGO Primo will load it and read it.

But that's about as far as I usefully get. I created a route that starts from home and ends about 50km away, after completing a loop to the north and back to that point 50km away. Whilst I can see it in Primo, with all its little yellow flags delineating the route, when I actually ride it, it goes a different but more direct route for some of the way, and then constantly tries to direct me back home, even though the finishing point is 50km away. The internal heuristics seems to ignore the plotted route and seeks the shortest path to the destination.

You can see this route here:-

The question is whether anyone has successfully used any external route creator that Primo will reliably read from start to finish, and all the waypoints sequentially in between?

I can create the same route manually on the GPS by entering in a long sequence of co-ordinates as the way points, and the GPS works fine, or perhaps more accurately, IGO reads it just fine.