It's a joke isn't it! How did you go about getting your defaults removed fkofilee?

Hi, I'm looking for a bit of advice please... I have recently checked my credit file on Noddle after being turned down for a loan and noticed that Orange c/o Lowell Telecommunications have registered a default on August 2014 for £25.77. I upgraded my phone in January 2014 with Orange. I stayed with the same network but Orange has now been taken over by EE so my contract is now with them. I moved house in May 2014 and during that time to now, I have received no correspondence from Orange/EE regarding an outstanding amount even though they have my new address. When I called Orange collections department, they advised me that the debt was for the final payment on my old Orange contract, even though I had made 2 separate payments on the last month of my contract to bring the account up to date before my new contract started. At no point have Orange tried to contact me either by post, phone or email even though my mobile number and email address have remained the same. I have written to Lowell Telecommunications and they have told me that they cannot remove a default from a credit file, only Orange have that authority even though they have bought the debt from Orange. I called Orange and they told me that the debt has been sold to Lowell Telecommunications so it is no longer their responsibility. I feel like I have exhausted all options to get this resolved. I need to apply for a re-mortgage next year and this default is really badly affecting my credit score so I'm going to have no chance unless I can get it removed. Does anyone have any idea where I can go from here to get this default removed? Many thanks in advance for your help! J