solved I think
I've found a solution if not the answer! I needed to recalculate the route once on the Garmin, then all OK. I'm using OSM from Talky Toaster, routable maps without contours. Not had this problem before, think they're the same version of maps on both Garmin and Basecamp.
I'm still trying to learn Basecamp, through after 2 years might give up, map source was so much easier, which is why I'm giving Tyre a go, but it's only on PC which is a pain as I'm a Mac user, needed to fire up an old windows laptop