HSE - Health and Safety Guidance 274 Part 1-3
The guidance gives practical advice on the legal requirements of the Health and Safety at Work etc Act 1974, the Control of Substances Hazardous to Health Regulations 2002 concerning the risk from exposure to legionella and guidance on compliance with the relevant parts of the Management of Health and Safety at Work Regulations 1999.
The guidance is in three parts:
HSG 274 Part 1-3
HSG 274 Part 2: The control of legionella bacteria in hot and cold water systems (PDF)