As far as I’m aware you can’t transfer maps bought on another platform over to View Ranger.I had the same, was using memory maps initially to plot routes (then transfer to old garmin 401) having bought the OS maps for that, then tried their app and it was poor. Went over to View Ranger and had to buy more credits to expose certain areas of map I wanted in OS. View Ranger is quality though, I use it on my rides sometimes if exploring, especially in the Surrey Hills. Have it strapped to the bars or top tube, and if you have a route planned you can follow and it will let you know when the next turn is coming up etc as you follow the route on the map. Downside is that you’re using the phone’s battery if you allow it to, the screen will be on constantly.