In this chapter:
"employment" includes—
(a) performance of work—
(i) under a contract of employment or contract for services (whether written or unwritten); or
(ii) as a minister of religion or as part of the duties of a religious vocation; or
(b) undertaking practical training as part of an educational or vocational course; or
(c) performance of work as a volunteer; or
(d) performance of work under, or because of, a sentence, including a community service order.