There should also be another tick box that says.
This server requires SMTP authentication.
Yes, that's ticked.
I'm coming to the conclusion that it must be the ISP although it's odd that if I use the BTYahoo setup I have no problems, only with Thunderbird.
Over the last year I've also found that it has become increasingly difficult to get on to the internet via BTYahoo at peak times - on Saturday evenings I've had to wait for up to three hours before I can connect to the home page, but it had got a bit better recently. There could be a connection.........no that's a most inappropriate phrase, but you get my meaning!
Thanks anyway, folks, I'll just keep trying.
GG