The Institution of Occupational Safety and Health (IOSH) are running a three-part series on Legionella, the latest installment looks at control methods and risk assessments of Legionella in water systems.
Read the IOSH article Good Practice: Legionella in hot and cold water systems.
It is also worth reading the Health and Safety Executive (HSE) guidance on the control of legionella bacteria in water systems which is available to download, for free, from the HSE website.