There should be a way for users to recover a forgotten Community login name.
The login page for Talktalk Community contains a link to click if you need help with your login name or password. But clicking on this link brings you to a page where only the password can be reset. There is no way to obtain help recovering your login name. The only method for recovering a login name seems to be to phone Talktalk. But doing that put me in a phone call of over 30 minutes with an agent who did not succeed in finding a way to find my login name. Eventually, by experiment I guess my login name.