Lowell Three Mobile

[cmr]

My son had a rolling monthly sim only contract with Three Mobile, he moved abroad and contacted them, they cancelled the contract and even had a refund from them. Fast forward, to 2 years later, he gets a letter from Lowell asking for money. They say that if he doesn't pay up, they will issue a ccj. My son doesn't owe Three or Lowell any money, he has sent a SAR to Three -no reply and one to Lowell who sent a load of computer print-offs, the last page simply says that he owes an outstanding balance with no explanation. My son still has screenshots of the conversation with Three confirming that the account is closed and giving him a refund. Where do we go from here? Three sold a debt that doesn't exist and Lowell are trying to claim for a debt that doesn't exist.Any advice would be much appreciated.