Groundwater will typically be too cold to support most pathogens

Inspired by your post I had a quick internet search and seemingly bore holes come under the Private Water Supply Regulations http://www.legislation.gov.uk/uksi/2009/3101/contents/made unless the water provider specifically owns them - by example Thames Water relies upon borehole water

Happy reading (I would check for changes to legislation given these regulations are 10 years old)

Adding another link http://www.filterclean.co.uk/Private_water_regulations.asp

Edited by user 27 August 2019 18:35:34(UTC)  | Reason: extra link

BS8580 makes the explicit point that risks are higher of contamination for water entering a system from a natural source, such as a river, lake or spring, or private water supply, rather than a treated and disinfected mains water supply.

The temperature of the water at point of entry to system (unless over 60 degrees) is irrelevant. Under 20 legionella doesn't perish, it simply doesn't grow. Unless you know the water course etc. in detail then assumptions can't be made.

I have always treated private water supplies as higher risk. As above, there are separate requirements around water quality and part of the risk assessment process should be an examination of the source water quality - including typical microbial content and growth risk factors such as nutrient content etc. The duties to check this sit with the owner of the supply but the owner of the water system needs to include additional control measures as necessary to check incoming water quality within their written scheme.