This thread has prompted me to have a look around to see what is available for my Windows 8.1 Phone, which as most of you are probably aware isn't much. I can't have ViewRanger or OS Maps for example.
What I have found is a Windows Phone-specific app called
GeoGPS and I've just downloaded a trial.
https://www.microsoft.com/en-gb/store/p/geogps/9nblggh188h2I've found apps for Android and iPhone that have the same name but they appear to be different apps from this one.
It includes OS maps from Bing that can be zoomed up to 1:50k and 1:25k, I have found, and I particularly like that my location doesn't get moved off-centre while I zoom in, like it does on the GPS to Grid Ref app that I have. It says that areas can be cached for offline use, though I don't know yet how big an area that will be. The compass won't work on my 8.1 phone, but I expected that as you need a Windows 10 Phone for a compass.
I'm already thinking it will be worth putting my hand in my pocket for, even if I don't use it to its full potential. It costs £8.39 which is more than the usual MS app but I think it will be worth it for the OS maps alone.