It depends on what browser you are using as to how you clear the history. If you do an Internet search "how to clear history" for the browser you have you will find how to do it.
Are you now getting the BT logon page and not the Yahoo log on page?
When you get logged on, go to the "Change View" at the right hand side. Click on it and click on one of the "Preview Panes".
Do not click on the "attachment view".
Is your inbox now showing correctly?