If a recruitment agency employs any staff, Employers Liability insurance is a legal requirement.
For businesses providing pure recruitment services, it's unlikely that Employers Liability insurance will be required for the workers they place. If the worker is under the supervision, direction and control of the hirer, or is bona fide self-employed, the responsibility for these workers will usually fall to other parties.
There are some circumstances, however, where a recruitment business has a responsibility to insure placed workers under an Employers Liability policy, and any recruitment business should take time to understand their exposure and arrange cover accordingly.