From the HSE:
"You are only required by law to have employers’ liability insurance for people who you employ under a contract of service or apprenticeship.
Whether or not you need employers’ liability insurance for someone who works for you depends on the terms of your contract with them. This contract can be spoken, written or implied. It does not matter whether you usually call someone an employee or self-employed or what their tax status is. Whether you choose to call your contract a contract of employment or a contract for services is largely irrelevant. What matters is the real nature of your relationship with the people who work for you and the nature and degree of control that you have over the work they do.
You may need employers’ liability insurance for someone who works for you where:
- you deduct national insurance and income tax from the money you pay them;
- you have the right to control where and when they work and how they do it;
- you supply their work materials and equipment;
- you have a right to any profit your workers make although you may choose to share this with them through commission, performance pay or shares in the company;
- you require that person only to deliver the service and they cannot employ a substitute if they are unable to do the work; or
- they are treated in the same way as other employees, for example, they do the same work under the same conditions as someone else you employ.
You may not need employers’ liability insurance for people who work for you where:
- they do not work exclusively for you (for example, if they operate as an independent contractor);
- they supply most of the equipment and materials they need to do the job;
- they are clearly in business for their own personal benefit;
- they can employ a substitute when they are unable to do the work themselves;
- you do not deduct income tax or national insurance.
However, even if someone is self-employed for tax purposes they may be classed as an employee for other reasons and you may still need employers’ liability insurance to cover them."
More details here:
http://www.hse.gov.uk/pubns/hse40.pdf