Credit not applied and my services could be suspended

Hello. I had some issues a while ago with upgrading/degrading and credit. This meant I was overcharged twice at a rate of £54 and not the agreed £26, but I got this sorted with virgin and I was told a credit would be applied when the next bill was generated. This bill came through without credit being used but it was at the price of £26 so I assumed credit would be applied from the next bill onwards. Now that the second bill has come through, £26 is being used from credit but I have been told that I still owe £28 from the previous month and if I do not sort this immediately they are going to suspend my services. I can't get through anywhere else and I don't want to be charged another £28 as that will mean virgin have taken £84 from me that I have never got back (only in credit). Please help!