Then actually, your situation is much more provable than your first post implied. In effect, you are 'living with' your parents, and that is a fixed address. There are many people on the forum who have gained a partner-sponsored visa while staying with family or friends. Nobody can give INZ evidence which doesn't exist - such as rental or bills which aren't due because of the way you're living. Your parents will write to explain what you are doing - and it's obvious that you have your private space as a couple within their property, which is the essential for showing partnership. Also, the fact that your partnership has already been accepted as genuine by INZ with the grant of a previous visa does indeed ease the way. As far as applying for partner-sponsored residence is concerned, it's proving the continued co-habitation for 12 months which is important. See old threads which have discussion and examples
https://www.google.com/search?source...10.Kw1gVkRjh_Q, and make sure to get a good variety of proof, e.g. mail, deliveries, organizations printing off their records of your contact details and/or each of you as the other's next of kin, etc. etc., to run right through the 12 months leading up to your application.