Neil, have a good look at Fatmap. It is a free App and has many different functions. My (military) son tipped me off about it and I'm impressed. You can even use one overlay to see which areas of ground are flat, another allows you to visualise slope, and you can even "fly" along ridges and ascents. I've found the aerial photography to be very up-to-date too. If I was more techie I would be able to do even more things with it.
Like you, I bought Viewranger maps when they were on offer and have transferred to Outdooractive so I can still use the maps I bought, but I am not convinced. I still use Anquet OMN on the desktop for planning.