I have been struggling with this for about 3 years - TalkTalk have been no help at all.
I have just managed to reset my password by doing the following:
Log on to your account and set up a reset alternative email when the “Just in case…” reset option screen comes up. You will be sent a confirmation of recovery details email to your alternative email address that contains a link and if you click on "Forgotten your password?" it takes you to a screen where you can enter your Lineone email address, and it then takes you to a reset screen. Follow the instructions. Wait five minutes and your password has been re-set.