If you are using a web browser try going to "change view" at the right hand corner and de-select "Conversation View".
It should be in the same section you found the "conversation view". If there is nothing there it may be that you are using the mobile version on your ipad and you may have to switch to the full version to get the setting.
I have presumed that you are on BTMail and not BTYahoo. Use this link to check if you are not sure.
http://diagnostics.bt.com/login/?workflow=EmailChecker&s_intcid=con_searchpriority_genericmail?s_int... Con_community:Email and Apps:L2:BT.com:BT Email Checker
I don't know about the iPad but the settings for that are usually at the foot of the page where it should give an option of full or mobile.