Overbilled and Ignored

I am on a 2gb contact. In December I was billed for 2gb and for 5gb AND for additional data usage that I didn't actually use - I never even used up the 2gb. I called to query and they said they would get back to me and they never did. After calling a few times someone acknowledged that I was overbilled in error and I would receive a refund in 7-14 working days for the 5gb and the data usage. That has still not happened despite numerous follow ups. In January I have now been billed for 5gb! and my contact is still supposed to be 2gb! On top of that I got a credit for December against my Jan invoice for 2gb and NOT 5gb and additional data usage.. No matter how many times you call or email, no one finalises anything. They have taken a lot of my money in error and I cant get it back! Are they just going to keep billing me and debiting my account when they are the ones that owe me money!?!