Thanks @sussamb . That helps to support my suspicions in the above post. I was almost certain that would have been the case, the more that I thought about it. My concern was the number of additional points that had been created in the Zumo on the section of road in question. If you haven't seen it - click the link that I have edited into my previous post. That would have been a real nightmare to sort out if that could happen simply by misplacing a route point.
Normally, if a single point is placed off road, Zumo and Basecamp both do the same thing - navigate to the road that is closest to the point, warn that you need to navigate off road and plots a straight line to the point (eg across a field). And indeed, when I rebuild the route on Basecamp with just the route points, that is what my Basecamp does using the OSM maps.