My flat mate recently moved out and our BT was under his name. i was looking to change the account and billing details over to my own but i am not sure how to?
You can`t do that. He has to close his account, and you have to place a new order in your name. This will give you a new account number.
you cannot change account holder's name. the account holder need to cancel the contract which may incur charges and then you or another flatmate need to apply for a new contract