So setting up my new watch and googling stuff, I discover you can't use Google pay on phone.
OK let's see if any of my accounts are supported by Samsung pay - great 2 area!
So I can just set up Samsung Pay on watch and still use Google Pay on phone - right?
Nope - on launching Samsung Pay on phone you have set as default or close app! WTAF!
Spend ages Googling including on this forum going round in circles until by chance fiddling with the watch I stumble on the solution!
In Galaxy wearable app add Samsung pay app to list of apps.
Then when watch has synced, launch Samsung Pay app on the watch and click add card.
It instructs the phone to open up the add card page in the app bypassing seemingly the set default app block.
Added card and saved and tested it today and it works.
Hope this helps others.