@Laki
Proof that someone is paying the broadband bill at a property is not proof of residence.
And broadband is not a utility. Look more closely at who is requesting this information, and what sort of proof they will accept, and I don’t think you will find a broadband bill is among them.
Set a Payment PIN on your account so that no-one but you can buy memberships on it.
Check your bank accounts monthly for any other unexpected payments to Now.
That way you can at least nip them in the bud, while you and Now figure out whose fault they are.
