Thanks for the replies.
Ancient Observer
Also, any "intermediate" box, such as BT/Virgin etc boxes might solve the issue, if he/you have them lying around.
This is the root of the problem. He's had about five BT 'Youview' boxes that keep failing. We think its because he uses those internet adapter plugs to send the broadband over the house electrical circuits. Until I can get in there and route an ethernet cable half way around the house to the BT Box, I thought it would be a quick temporary fix to use the smart TV App.
Netstruggler
Yes, he's tried both, typing in the username in lower case and using the email address.
Good shout re USB keyboard. I'll tell him to give it a go. He's nearly 83 but is pretty good with computer tech normally.