Hi Sue,
We use Dantek for our legionella risk assessments. Because we are low risk with four almshouses and no water tanks, they recommend visiting every 5 years.
In the meantime I currently test the hot and cold water temperature in each flat on a four-weekly basis and record the results. Dantek did their five year visit this week and told me I only need to do it every three months!
You can buy a water thermometer on Amazon.
Kind Regards,
Sally