All landlords have a duty of care to the tenants housed in their accommodation. The HSE requires that landlords carry out satisfactory risk assessments for Legionella. Consequently, there is also an ongoing responsibility to maintain control measures to minimise risk. Further guidance from the HSE regarding your responsibility as landlord in relation to Legionella can be found here
The law states that if you are a landlord and rent out your property, including a room within your own house, you have legal responsibilities to ensure the health and safety of the tenant by keeping the property safe and free from health hazards. This is stated in Section 3(2) of the Health and Safety at Work Act 1974 (HSWA).