This will have been a problem for as long as SA has existed, but it only crossed my radar today, so I though I'd share and see if there's any mileage in taking it further.
Where an employee has a week 53 salary, they get an extra "slice" of PA to ensure the net pay remains the same.
If employee is under SA, the result is an underpayment of tax which is collectable, and HMRC internal guidance confirms this is so.
Surely there's something in a Taxes Act that says all taxpayers should be treated equally; even if HMRC try to argue that the PAYE treatment is "concessional" I struggle to see why this concession should only apply to a particular type of taxpayer?
The amounts involved are small, but it's the principle I feel miffed about.