Thanks Stpehen. I've just tried using the Snap Route to Map feature on that route and it worked for me (with "Walk" as the Activity and "20m" as the Proximity). Did you have the Activity set to "Bike"? It's possible that there is a section of the route where bike routing is not available. Usually routing on foot is more permissive than routing by bike, so its worth trying that if routing by bike doesn't come up with anything.
Many thanks for that. Very much appreciated. I have also found your method successful. Will remember that in future. Stephen