Landlords have a legal responsibility to ensure that the property that is being rented is safe for habitation. This responsibility extends to the water system and landlords should ensure that water is both suitable for drinking and also free from potentially fatal Legionella bacteria.
It is therefore sensible to perform routine microbiological sampling of water system prior to occupation by a new tenant to show this duty of care and prove to the tenant that on receipt of the property the water system was clean and safe.This also applies to people who own and rent out holiday homes and accommodation.

Health and Safety at Work Act 1974Control of Substanses Hazardous to Health Regulation (1994)
ALCOP L8 (The control Of Legionella Bacteria in Water Systems)