Vodafone default - 2 years later !!

Vodafone have put a default on my account from over 12 months ago. I had an account for my daughter and waited for the contract to expire then cancelled the account - all confirmed from Vodafone. Then a month later another bill, contacted Vodafone again and was assured that the account was closed and there was no monies due to them. The next month yet another bill - hours and hours on the phone to be to the same. The next thing to happen was a letter from a debt collection agency I spoke to them and they put the account on hold. I explained that I had a confirmation e-mail of a live chat that the account was cancelled. That was last October, I contacted the Sun on Sunday and my complaint was printed - in no time I received a phone call from the directors office at Vodafone confirming that this was all sorted out and that would be the end of it!! Then this year in August I had a notification that there was a new entry on my credit file - Hey ho a default from Vodafone. I queried with them again they told me this would be removed and it would be off my account within 4-6 weeks. This is still on there - how can I get rid of it ?:mad: