Under general health and safety law you have to consider the risks from Legionella that may affect your staff, or members of the public, and take suitable precautions.
As an employer or person responsible for premises you must:
Identify and assess sources of risk
Prepare a scheme for preventing and controlling the risk
Implement and manage the scheme - appoint someone to be responsible
Keep records and check that what has been done is effective
Notify local authorities if you have a cooling tower(s) on site
Examples of systems that present a risk:
Cooling towers
Evaporative condensers
Hot and cold water systems
Humidifiers
Spa baths
The above is not an exhaustive list; any water system where water droplets may be created can present a risk from Legionella.