@StefanHummelink Thanks, I think you've pointed me to the problem. I had habitually used the Here map & Overlay because I previously used a Garmin Satnav and it matches the Garmin routing. Now I'm using the phone app, and I'm guessing that the app uses Open Street Map? Certainly if I plan the route using OSM (or Tom Tom) the app routing matches what's planned as far as I can tell. I just assumed that the phone app would understand whatever was done in the web planner.
An additional workaround, which I may also use, is to use the Expand function, which adds in as many extra waypoints along the way as you want. Presumably these would enforce the routing - I'll find out tomorrow!
Thanks for all the help