HMRC have amended chapter 1 paragraphs 9, 10, 11 and 12 on pages 13, 14 and 15 in the employer helpbook Employer Further Guide to PAYE and NICs CWG2 (2012).
It covers what the employer should do where a mistake has been made in the amount of National Insurance or PAYE deducted.
The updated version can be downloaded from HMRC’s website.