There is no risk of legionella from turning off hot water devices overnight and at weekends, however, consideration is needed for longer timeframes like a week +.
We use Rock Compliance, who are experts in water hygiene, control and legionella risk for our legionella compliance. This is what they had to say on the subject:
“Legionella grows in an optimum water temperature of between 20 and 41C. It accelerates at 37C. The current legislation and advice state that if water does not move for >1 week, you need to flush once per week for 4-5 minutes. Zip taps that have been powered off over a weekend will get back up to temperature and boil the water as it leaves the tap. The combination of standing for <1 week and the high temperature means there is no risk from legionella and flushing is not needed. When the zip tap is used, any initial non-boiling water is left to drain as is per normal usage.”
Andy Smith, Rock Compliance