I strongly endorse P6 Driver's line on the ESC A19 idea. Last year I got a £6K bill for underpayment in previous years - shock horror - and I am PAYE only.
Basically it says: "You've had the correct info but got it wrong. That's your problem not mine, please write off the debt iaw some regulation."
Result! Bill cancelled - and I too found out about it from Pprune!!