Found the solution here right down at the bottom How i got Safari to have enabled WebGL ag… - Apple Community
For people who migrate rather than do a clean install we sometimes get unwelcome things transferred over including in my case a hidden message to disable WebGL dating from when it was WebGL 1. If you search for a file called com.apple.safari.plist and delete it (to be safe I moved mine to the desktop) the problem is completely solved. Problem is I don’t know what else I have thrown away in the process!
The map in CityStrides now works perfectly. I have spelled it all out in case someone else has the same problem.
Thank you very much James for all your help with this - I really appreciate it