Having registered as Jack_M, I cannot login as the underscore is not allowed, so I have had to re-register with a new name and email address
That’s news to me - my @Midnight_Voice handle works fine.
Perhaps there’s another reason why @Jack_M isn’t working for you - what error message are you getting when you try to use it?
It's working now! Used to say that username contains invalid character or words to that effect