I got a very swift response from Viewranger Support, here is the reply
"No sign of a new device ID being allocated, which suggests the app's settings were not lost.
So I'd check for any disk/space cleaning apps, if you have one remove it or tell it to leave the ViewRanger folder alone.
Or if it occurs again, try closing; using the Settings app, Apps, ViewRanger, Force stop (or the phone's taks manager to close the app); restart; see if it still needs a key"
I'm not sure if my tablet has any disk cleaning apps, I will have a look. I think they are suggesting it was glitch on my tablet rather than a Viewranger problem