You definitely need something in writing from the landlord (letter stating that you are permitted to live at the accommodation/address with your partner, plus signed tenancy agreement or rent book would be best). As far as I'm aware there are no set rules on this. You need to satisfy the ECO that you have 'suitable accommodation' that's all.
I'm sure your name doesn't need to be on the tenancy agreement though.
So...Letter from landlord, plus tenancy agreement with at least your partner's name on it.
