CCJ Dispute

I wonder if anyone can help.
I was living in a rented property with my ex partner back in 2012.
I moved out of the property in November 2014, this was my first time living on my own so I was unaware I had to notify the water company that I had moved out.
The water bill was in both of our names.
I went for a job interview about a year ago and they did a CCJ search on me.
I had no idea but it came back stating I had one registered from Welsh Water.
My ex partner stopped paying the bill as soon as I left the property and was receiving letters from them.
He was living at the property for the next 2 years.
I have contacted the company on lots of occasions to argue the fact I wasn't living there and my ex never informed me of the bill but they advised that it's basically tough and I have to pay.
I have now decided to put a formal complaint in as I have been speaking to people who have advised that I shouldn't be liable as both of our names were on the bill....
I did pay over £2000 of it as I need to get the CCJ off my credit file but I am now struggling and I don't see how it's fair for me to pay for someone else's water bill.
Hope someone can shed a bit of light.
I have tried reading through the Water Industry Act but I cannot make any sense out of it!
Should provide one for dummies I think!


Thanks in advance.