worldpay
In January I logged into HMRC for my self-assesment thingy. At the end I was told that I had £123.45 (not the real amount) outstanding, so I clicked the link and paid it with Worldpay. I got a confirmation email from Worldpay with a reference number. Last week I got a letter from HMRC demanding exactly the same amount so I rang up to query it. I was told that they could not use the Worldpay reference number to check it. D'uh!