They haven't sent a letter no, I can see it in my end of year tax return though via Gov.uk. I'm not sure how to challenge it, any advisor I've spoken to says it is correct but won't elaborate on how they've arrived at their conclusion.

In the last tax year I received a PPI payout, the issuer paid the statutory 20% on the simple interest to HMRC as required. The total interest paid out in the claim was around £1300, my issuer paid around £265 income tax. I have tried reclaiming this from HMRC using R40 but they claim I owe them more in tax as I have received too much savings income? I am aware that I am entitled to receive £1000 tax free savings income each tax year, so I thought I'd be able to claim back the difference? Is it not the case that HMRC can only claim tax on the difference between £1000 and what was actually received, so circa £300 and refund the remainder back? Or am I barking up a wrong tree?