Good news!
Bar one very odd glitch, the method suggested by GrumpySimes, where you use MyRoute’s waypoints (ie ‘hands’) to pin the route into place, works a treat.
When I create a route, I place the first point about a mile down the road, which gives a modern GPS device somewhere to aim for, after which the route should run normally. When I fired up BMW’s Connected app, it (quite rightly) pointed out that I was not at the start point, would I like to go via it? I said yes. For the first 10 to 15 miles, all was well. Then, at the first major T-junction, my XT and MyRoute Navigator app, both said, turn right…. The BMW app said, turn left. My heart fell! The BMW app had altered the entire route! The mileage and arrival time had both altered. In short, bugger!!!
Not to give up, I stopped the BMW app, and fired it up again. I then selected: Go direct. The app recalculated, the mileage and arrival time matched the other two devices and the shape of the route was correct. Away I went. Despite going off-route on purpose, the BMW app recalculated perfectly. No more, going back to pass through points or roads missed.
My thanks to GrumpySimes, for all his help.