Thanks Erel.
That has solved this particular problrem, I now have this (not uncommon issue):
Undefined symbols for architecture arm64:
"_CBCentralManagerOptionShowPowerAlertKey", referenced from:
l936 in GoogleMobileAds(flat-arm64)
"_OBJC_CLASS_$_CBCentralManager", referenced from:
objc-class-ref in GoogleMobileAds(flat-arm64)
"_OBJC_CLASS_$_CBUUID", referenced from:
objc-class-ref in GoogleMobileAds(flat-arm64)
"_OBJC_CLASS_$_SFSafariViewController", referenced from:
objc-class-ref in GoogleMobileAds(flat-arm64)
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
I have tried looking to see what other people have done but I have not managed to resolve it.