Changing Apple ID - Beware!
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So before changing your Apple ID on the Mac, especially from a 3rd Party email address (like talktalk) where there is no way to go back now Apple do not allow 3rd Party addresses, make sure that the phone and iPad are both signed out of ALL iCloud services. The one I left was Find My Phone/iPad.
Else your phone/iPad will nag you to sign into the previous ID account which no longer exists! Had Apple allowed me I could have altered it back and closed all the services down.