If it isn't the supplier you picked with helloBill, we didn't set it up. When someone new is going to move in, the supplier already at that address is sometimes told by someone else involved in your move, such as your landlord, your letting agent or the company that handled your tenancy paperwork. The supplier often opens an account in your name, so the home isn't left on an account with no name on it.
You don't need to sign up to anything with them. If your new supply starts on the day you move in, you shouldn't owe them anything. You may get a final statement from them, but it should show nothing to pay. The new supplier you picked with helloBill will be in touch to confirm your switch, and the existing supplier is told automatically and closes the account. If there's a gap between moving in and your new supply starting, the existing supplier may bill you for the energy you used during that time.
Take a meter reading on the day you move in and send it to both suppliers: the existing one and the one you picked with helloBill. It's your proof of what you used and when your new supply started. If you have a question about a bill from the existing supplier, contact them directly. If you're not sure when your new supply starts, email [email protected] and we'll let you know.