The other tenants are friends of mine and are happy for my partner to move in. We split all the bills other than council tax between us. We currently pay £35 each in our rent towards the council tax.
I didn't think that there was anything law wise I could use to back up an argument to pay less but thought I'd check. As you say it's up to the landlord to decide what they want to charge and us to accept it or move on.