I have used multiple PayPal accounts on 123 Reg in the past, how do I know which one you are using?

This article refers to payment via PayPal.

The last valid used PayPal account will be stored within your payment priority control panel. This is the account that will be used for all future transactions unless you cancel your PayPal arrangement or you make a new purchase via your 123 Reg account using a different PayPal account, which will then become you active PayPal account.