I should have said, it was on airplane mode, and I was still!
So much for my troubleshooting skills
As others have said it's beginning to sound like some sort of software problem, or the app just doesn't like your model of phone.
It might be worth trying a different app to see if that works for you. I use Viewranger - although they've been taken over by OutdoorActive I think you can still install the Viewranger app. It can give you an OS grid ref for your location amongst other things. You need to change a couple of display settings - change grid to Great Britain, and coord-type to National Grid - then from the map view, tap the compass to get your location, and the grid ref appears top left of the screen.