tax man wrote to me advising he's checking the tax I paid for student loan's

[Papavar]

I did a similar thing when the tax forms changed a couple of years ago; I missed the box you tick to confirm you should be making repayments. I gave HMRC a call, explained that it was just a mistake and asked them to send me a bill for the money I owed. They did, I paid it. They closed the investigation. Simples.