Pay and leave benefits are calculated on a pro-rata basis. Pro-rata means in proportion to the hours worked. This means that part-time workers should receive a pro-rated proportion of pay and benefits received by a comparable full-time worker, according to the proportion of full-time hours they work. These terms should be written into their Employment contract.
A comparable full-time worker is someone who:
works for the same employer and at the same place (where no comparable full-time worker works at the same place, at a different place)
works under the same type of contract (eg not some working under an apprenticeship agreement)
is paid fully or partially by reference to the time they work
performs the same or similar work, and
has similar levels of skills, qualifications and experience
Holiday entitlement should be pro-rated but it is permissible to only give bank holidays that fall on the normal days of work of a part-time staff member.
For pay, it is generally a straightforward exercise to pro-rate the entitlements.
However, some benefits cannot readily be pro-rated, such as a company car, insurance benefits or gym memberships and employers sometimes simply withhold these benefits from part-time staff. For benefits that are withheld from part-time workers, the employer will have to show that this is objectively justified. To be an objective justification the reason for the different treatment must be:
necessary to achieve a legitimate aim or business objective, and
a necessary and appropriate way to achieve this objective
This may require showing that less discriminatory alternatives were considered but were not practicable (eg because the costs involved were disproportionate). One route to consider is providing a pro-rated cash equivalent of the benefit.