CCJ on my credit file for debt that isn't mine?

From July 2014 - Feb 2015 me and my family lived a private rented house. It was all in my mothers name. We moved out and that was it. A few months ago after being rejected for credit I checked my credit record online and found that I had a CCJ against me of £600 for a water bill from the house mentioned above? Now they're constantly sending me letters saying they are going to inform my employer of the debt and get it taken from my wages. I have asked what I need to do to prove that the bill is my mothers and not mine and they said they needed a council tax bill showing that the house was in my mothers name however when we went to the council they would only give us a print screen of their files and the water company have said this is insufficient. The only other evidence that can prove that it's not my debt is the tenancy agreement but she doesn't have that anymore and does not speak to the old landlord. I need this removed from my file as I want to eventually get a mortgage and with this I have no chance. It's really depressing me.

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