I solved the issue, well sort of.
I tried Reaper in a Windows VM on the same Macbook Pro and it worked perfectly.
So then I tried deleting and reinstalling Reaper on macOS and still no good ...
Then realised that deleting the app doesn't delete the preferences.
Deleted the folder ~/Home/Library/Application Support/REAPER
Relaunched Reaper and set up the control surface again ... and it works perfectly.
Very strange, and the problem solver in me will be frustrated that I didn't find the true root cause ... but ultimately I am happy that it is now working as expected!
Thanks for you help in leading the way, without the link you provided I wouldn't have discovered that App Support folder!