Hi everyone. Sorry for the delay in replying to you all.
Sky Customer Services fixed my problem over the phone with me.
In my case, the person who took my original order really messed up my address on my Broadband order but it was right on the TV side of things. (Apparently they're two different systems at Sky's end.)
Once my address was corrected, the Customer Service agent added confirmed the email and security question were correct.
Next I completely deleted (not offloaded) the Sky Go app on the Apple TV and reinstalled. This time it worked first time and has been fine ever since.
I have the Apple TV 4K, if that's relevant. I would add that the agent hadn't heard of any problems with Apple TV and the Sky Go app, but she did the above steps anyway and it worked.
Hope this helps you guys out.
