I am logged in as rmse047071 and can get the 1:50000 maps on GPS but not on the maps associated with the walk descriptions?
Some confusion here. To be clear,
there are no OS 1:50k maps on the GPS planner, only the Walkhighlands-own 1:50k maps (and OS 1:25k maps).
On described route maps, you should get the genuine OS map at both 1:50k and 1:25k when logged in, so they are better than the GPS planner map at that scale.
What are you getting on your route maps?