This is interesting as about a year ago I had to contact HMRC as I wasn't convinced my employer was paying my tax.
He owed me two months' salary and I hadn't had a payslip for about 6 months.
When he did pay me, he paid me the amount that I expected after tax. He never gave me a P60.
HMRC assured me that, even if my employer hadn't paid over my tax, there was no way that I would be liable for it.
I resigned, dear reader