Let me see if I can address at least one of your concerns today. You said, "It is likely that my shower head is probably the main source of NTM and / I live near the coast. The use of regular bleach by soaking the shower head has not been proven and here in the UK labs cannot test for NTM."

According to the National Institutes of Health, a 30 minute vinegar soak will effectively eliminate NTM< (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3940030/)
"Effective and economical mycobactericidal disinfectants are needed to kill both Mycobacterium tuberculosis and non-M. tuberculosis mycobacteria. We found that acetic acid (vinegar) efficiently kills M. tuberculosis after 30 min of exposure to a 6% acetic acid solution. The activity is not due to pH alone, and propionic acid also appears to be bactericidal. M. bolletii and M. massiliense nontuberculous mycobacteria were more resistant, although a 30-min exposure to 10% acetic acid resulted in at least a 6-log10 reduction of viable bacteria. Acetic acid (vinegar) is an effective mycobactericidal disinfectant that should also be active against most other bacteria. These findings are consistent with and extend the results of studies performed in the early and mid-20th century on the disinfectant capacity of organic acids."

Some members tie a plastic bag of vinegar over their showerhead every month or two and let the head soak for 30 minutes or longer. Others remove the head and soak it. The purpose is to disrupt the biofilm where the bacteria congregate, which takes some time to grow, so more frequent treatment is not required.
Further, some water experts recommend running the hot tap for a minute or two before stepping into the shower to "flush out" any bacteria congregating in the pipe.

I hope this answers one of your concerns.
Whe else can help our colleague with answers?
Sue