Thank you very much for your reply. It has been very helpful. However, on meeting with the water treatment company yesterday, he has now advised us that our samples after a year returned a reading of nearly 6000 cfu/mL, and not 825 cfu/mL as we had originally stated. If we chlorinate more regularly we could keep the level to below 1000, cfu/mL, but this would also rot the pipes. Does this new reading alter the advice that you very kindly offered in your previous email?
Chlorination is ineffective long-term against Legionella. Note that there is no clear-cut need to disinfect your mansion. Keep in mind in your neighborhood all large buildings and 10% of homes are colonized with Legionella. None of them are undergoing disinfection, nor should they. If you must disinfect, copper-silver ionization is effective but maintenance is necessary.