I’m trying to find a way to view multiple GPX tracks on the same map at the same time, with off line maps, ideally OS maps, on an iPhone. I can get some of the way there with lots of tools, but can’t anything that does it all. Does anyone know of anything that can do it?

Back Country Navigator will do that on my Android phone.

Viewranger allows you to view multiple routes on an OS map. I use Android but assume its the same functionality on an iPhone.

I have many riding routes saved in Ride with GPS.

I export these routes as GPX files.

I import these files into Google Maps (Maps > Your Places > then I create a Map and Create a Layer for each Route

Then I can view either one of multiple routes in the same map.

I assume if I download the map I can view all these offline.

Yep, viewranger on iPhone. Loads maps from Dropbox too, so I can source them on the desktop machine and chuck em on there and load em into the phone.

Memory-Map lets you view multiple gpx also.