Handed over wrongfully!!!

I got a call from virgin active consultants saying I owed them for the past couple of months. I tell them that to my knowledge I don't owe anything because when I joined the consultant had told me that there was a promotion running for that month. I call into the branch and they tell me that they know nothing of that promotion and the the consultant was no longer working for them. I was told I am liable to pay cause I signed a contract. Now the contract I signed had alterations made by their consultant that no longer works for them. I have now been handed over because of this thus giving me a bad credit rating. Virgin active has been nothing but a pain and by the look of things no one there wants to get to the bottom of this. Is no one going to help me.???