Yes - every year and at the start of every tenancy. A “competent person” must identify areas where water could lie for long periods of time and allow the health-risk bacteria to grow.
Legionella bacteria is commonly found in water. The bacteria multiply where temperatures are between 20 and 45 degrees centigrade and nutrients are available. The bacteria are dormant below 20 degrees centigrade and do not survive above 60 degrees.
Legionnaires' disease is a potentially fatal form of pneumonia, contracted by inhaling airborne water droplets containing Legionella bacteria.
For more information on Legionella and your responsibilities, please visit the Health and Safety Executive website.