Well I tried the OS app and ViewRanger together again today. I started the OS app first on my phone and set it to record a route. Once I was happy it was recording correctly and leaving a trace, I started ViewRanger as well to follow a route. Again I checked it was recording correctly and lotting a trace, then double checked the OS app. About 100 yards up the first hill I checked my phone and, lo and behold, both apps had stopped tracking. The off/on trick worked as before and every time I checked them, both apps were working fine from then on.
When I got to the the end of my route, ViewRanger had a good trace but the OS app had only recorded the last 9km or so although I checked it was tracking throughout.
Needless to say, my Garmin was fine as usual.
Incidentally, I used VR on its own on my Christmas day walk and it was absolutely fine, as it has been every time I have used it on its own.
Subsidiary question, I flashed up the OS app on the bus to the start of the walk and noticed that the location icon just ran of the screen as the bus went along- why doesn't the app keep the map centred on your current location - or have I missed a setting somewhere?