I've just checked a PAYE account back to 6th April 2017, following a claim from HMRC that the client was £4600 in arrears. I've found all 19 payments - including the Class 1A NIC - matching the RTIs as on the Government Gateway. There's no breakdown of the supposed debt in the HMRC letter. There may be a bit of interest - but even that's a bit weird because four of the seven months for which there are interest charges were paid on on before 19th - and, anyway, it only amounts to £62.
If the client owes this money, it's definitely from before April 2017.
If I click on a payment, it's allocated to three or four different months for no logical reason.
Does anyone else find this site totally unhelpful or is it just me ?