For anyone that's faced this issue I found a fix that works a very consistent basis!
Unzip your game somewhere, I'd recommend in a separate folder
Download googlemobileadssdkios.zip from https://developers.google.com/admob/ios/download
Open the file to unzip it
Once you've unzipped it you'll get a folder with a file called GoogleMobileAds.framework, take that file and put it in your game's folder (the one containing your .xcodeproj file)
Go into Xcode, click on your project and go to Build Phases and expand Link Binary With Libraries
Click the + icon and click Add Other...
then go into your game's folder and select the GoogleMobileAds.framework file and click Open
And that's it!
If you run the application now it should build with no problem