Welcome fellow brit ^_^
Trial and error.try playing around with what you got first then if needed buy some smaller powerful power heads and place lower down.
What i would be wanting is no sand blowing all over.
Enough flow for each coral i have ( may need re- place some of the corals after moved power heads ,depending how long they been there and if encrusted onto the rocks/ light they need etc etc.
Would want 1 or 2 power heads facing upwards for gas/ exchange/surface agitation.
Would want power heads to be hid as best as possible.
But like others have said so many factors that dictate whats happenning in our tanks.
Is the cyano getting phospates from the rocks or from excess feeding/ rodi filters need changing/ any natural predators of it/ refugium or no refugium that out competiting it or not plus much much more.
Split each bit into managable segments and after each adjustment wait a little to see if has made positive or negative impact as if change 3 things at once and makes it worse,we maybe wont know which affected it either way.
But good luck ^_^
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