O2 fraud and Lowell Financial Ltd

In January I received a bill from O2 for a new mobile phone contract which I had not taken out. The letter was addressed to my name but had Flat 8/3 on it when I stayed in Flat 8/2. Thankfully the postman put it through my door as he knew my name. I contacted O2 and told them I had not bought a new phone and this was a case of identity fraud so they cancelled the account. I informed my bank as well and they have been very good at keeping an eye on my bank account. I moved house in April. In August I received a letter to my new address from Lowell Financial Ltd saying they had bought my debt from O2 which was £1115 and I was to now pay them. The debt was for a mobile phone contract for an address at Flat 8/3. I told them I had never stayed at that address and the account had been cancelled with O2 as it was fraud. Lowell said they will need to investigate this and it will take up to eight weeks. I have since checked my credit file and it says I have an account in arrears which is with Lowell Financial Ltd. I am not happy about this being on my credit file as I am not responsible for this debt. How can this be removed and how long will this affect my credit rating?