Brilliant replies all. You have helped a lot. As to whether FIT or GPX is better, I guess I'll try both to compare. Probably what I experienced with my old Garmin 800 towards the end of a long ride, i.e. loss of turn by turn, FIT file showing a smaller and smaller on screen track, hard to view turns at roundabouts, etc, was due to too many track points on 600km file. On my next long ride I will split the ride into shorter sections, and join together at the end for Strava purposes.