The material in this paragraph must be read bearing in mind the change in the NIC treatment of termination payments after 5 April 2018, for which see ¶1006-612.
Many of the old Schedule E income tax cases revolve around determining the source of the payment. The same principles apply for Class 1. If the payment can be sourced to something other than the employment, it is unlikely to be earnings. Where the Government has had good reason to do so (e.g. raising revenue for the NI Fund), it has changed the sourcing rule by a deeming provision.