SOLUTION - I deleted the ‘user data’ and ‘cache’ on Google Play Services and it immediately fixed it.
This was AFTER Nothing’s own customer support told me there was nothing(?) I could do and it was a problem with Google themselves. (Which is was, however in my IT support days, we wouldn’t have been quite so quick to give up!!)
This could have been a MASSIVE coincidence, however reading up a bit, I believe my play services didn’t quite know which phone to sync with.
Anyway, thanks for everything, hope this helps someone else!!