UK Landlord Forum / Life

Hi
I am in need of some advice with regards to the room I am renting in a HMO house. I have been living in the property for 63 weeks, most of that time there were 4 other students living in the building, and for that period of time i have been charged the full amount of council tax as the students are exempt. Now where the problem arises is that I am in receipt of benefits and with the building being a HMO the tax bill is in my landlady's name I received no help with the costs. When I told my landlady at the start of the tenancy that I would not receive any help she said there was nothing she could do. When i told someone recently about my situation they said that I should look into it as they did not believe that she had the right to charge me anything as the bill is in her name. So I called my local tax office and two different people told me that no if she had wanted to charge me council tax she should have added it to the rent. My tenancy agreement does state under the tenants obligations: "To pay all charges...and council tax or similar tax". Now the people in the tax office still seem adamant that she should not have taken any money off of me and one said that as she is responsible and the only one who can be responsible for the council tax on a HMO that it shouldn't even say that in the agreement. And to top things off my tenancy agreement is a photocopy that has been modified and it states at the bottom that the document should not be copied or modified.
So I am now wondering is the tenancy agreement valid? and should I have been paying council tax?
Any help would be really appreciated.

Thanks