Best 2 Serve

I have ordered a disk from best 2 serve (HP's prefered partner, apparently) and I have paid for it about a month ago by bank transfer through santander. Best 2 Serve say they cannot trace the money and their payments department insist that they want a copy of my bank statement (with account numbers) from when I took the money out. On my statement it doesn't say where the money has gone just that it has been taken out of my account so it's not much proof that it's gone to Best 2 Serve. I have given Best 2 Serve information from the 'Telegraphic Transfer Received' document that I got from the bank when the transfer took place thinking this would give them enough information to trace my payment but they say they still can't trace payment from the information I have given them and are still after my bank details. Why do they need to see my bank statement if I have paid for something by bank transfer?