I'm not in the sector but so long as you can justify it then you're good to go with whatever cleaning frequency you wish. You've got to show that the frequency relates to the tendency of the system to foul. I'm sure you've checked HSG274 Part 1 although that has a disproportionate focus on fill-pack.

The question is 'will this decision stand up to scrutiny?'.

You could use microbiological sample results as part of that evidence but I'd also suggest photographs of internal inspections and your logbooks/records to demonstrate good control over all the other precautionary measures e.g. showing that a reduced cleaning frequency has not had a negative effect over the amount of chlorine/bromine used or its ability to clobber bugs in the system.
Its a bold move to step outside of 12 monthly minimum but not an illegal one.