Q: phone keeps asking for other phone passcode

I bought a new iPhone 6S Plus and once I had updated IOS I was able to back up from my older iPhone 5 via the iCloud.    I now get a 'follow-up' in settings.  Which says -

'Finish changing your passcode.'

I click on this and my phone asks for my Apple ID passcode. I enter this and then the phone asks me for my passcode.  I enter this and then my phone gives me this message -

'Enter Passcode for Other iPhone'

'Access to your account is protected by the passcode for "Mark's iPhone5".  Your passcode is encrypted and cannot be read by Apple.'

I enter my passcode for my iPhone5 and get a message saying "Incorrect Passcode. Enter the passcode for your other phone".  (The passcode I have entered is definitely correct and id the same passcode I use for my iPad.) I enter my passcode again and get the same message.  If I enter it a 3rd time I get a warning and have to start from the beginning again.

There is an option to enter a passcode from a different device, so I choose my iPad and enter the code.   When I go back to settings the follow up flag is still there and I am right back to the very start.  I have done a reboot, but the follow up flag is still there.