Simple Assessment Tax Letter

[AceOfHearts]

Last week I received a simple assessment letter stating I underpaid income tax last year. I logged onto the gov website as it asks, but online it says that I have paid the correct amount and it doesn't have any option to pay or give the letter reference number.

I have checked the calculations and the letter is correct that I have underpaid, but trying to call HMRC brings me to a dead end and just tells me to go check my government gateway again

Anyone else had this?