Literary Gal - do you have a paper Ordnance Survey map with the scratch off code for access to the same map digitally?
If you do, your best bet will be to use the
Ordnance Survey Mobile Download service which will allow you to view the digital version of your paper map on your phone. I believe the cost of the Ordnance Survey app is free and, seeing as you've already paid for your paper map (with scratch n sniff code), you do not need to pay again for the digital version. (The digital map isn't viewable via any other app than the OS mapping app so won't work via ViewRanger.)
If you don't have the paper map scratch off code then your best option will probably be to sign up for either Ordnance Survey's or ViewRanger's digital / online mapping service - both of which will allow you to view 1:25k Ordnance Survey maps on your mobile device.
Like sussamb says, you have to pay for Ordnance Survey mapping (either by subscription or individual map/tile purchase) but OSM (Open Street Maps) are free to view on both of these (and other) apps. The thing is, they don't look like a "normal" Ordnance Survey map and might be a bit confusing if you just want a visual like-for-like spot-check confirmation of your location.
Hope this helps.