The definition of an employee for the purposes of health and safety legislation is very different to that used in other aspects of employment law.
The term has been subject to a number of court cases and interpretation and explanation by senior judges.
Employment for health and safety purposes does require an exchange of money and nor does it depend on the arrangements made for paying tax or National Insurance. It is entirely dependent on a connection with a work activity and the person in control of that activity.
An owner of a business may think that by employing someone as a nominal sub-contractor that they are not their employer. But if they provide materials, or give instruction as to how the work is to be done, or are themselves involved in the process they are likely to be considered in control and the court will decide that there is a master and servant relationship (employment).
Another factor used by the courts in deciding whether the relationship is one of employer and employee is to consider whether the worker can decide when to do the work, take time off when they wish, or send someone else to do the work. If the worker cannot exercise these options at will they are likely to be considered an employee.
See also: Agency Workers; Temporary Staff