So you come home one day and there's a bill on the doormat from a utility company, but it's addressed to a former flatmate. Or even the landlord. Do you need to do anything about it?
Let's be clear, you're not responsible for paying bills that relate to former flatmates. If they've left without paying a bill they risk damaging their credit rating, which will affect their ability to get a loan or mortgage in the future. If they left a forwarding address you should send it on, or ask your current flatmates to get in touch with them about it. But if it remains unpaid, at some point the utility company will come knocking, so it's best to get on the front foot and get in touch with them first.
Confirm the date the previous flatmate left with the landlord, and provide this information to the utility company, along with any forwarding address, meter readings and any unopened bills.
If the bill is addressed to your landlord, get in touch with them as soon as possible to let them know. You're not going to be held responsible for bills in the landlord's name, so it's up to them to sort it out.
Of course, when it's your turn to move out, it's your responsibility to inform the utility providers promptly. Provide them with meter readings and a forwarding address, because you're unlikely to want to leave that kind of mess behind you.