HMRC voluntary payment paying-in slip

[selwonk]

I have underpaid tax by a small amount due to a taxable benefit changing. The HMRC have now put me on an emergency tax code and suggested that the easiest way to sort it is a voluntary payment.

I need a paying-in slip but I can't seem to find one online. Does anybody happen to know where I can find one? All I can find is self-assessment ones.

Thanks in advance.