HMRC have published guidance on their website explaining when a marginal item of pay can be included in an earnings period later than the one in which it was paid or treated as paid. The guidance also confirms that payments by of earnings, such as in the form of shares or securities, are not treated as marginal items of pay.
The guidance, which replaces that contained in Chapter 1, para. 4 of booklet CWG2, Employers' Further Guide to PAYE and NICs, is available on the HMRC website.