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Simon, just use a pressure washer, with a regular downstream chemical injector.
Use a soap tip {BLACK} and apply water to all plants with it first, then hook up to a jug of pool chlorine and it will apply about 10-12% chlorine automatically.
Leave it set, but don't let any areas dry out. You may need to recoat it again after 10 minutes?
Then after say 14 or 15 minutes, put on a pressure washing tip { Green or Yellow } and clean it back off.
Then just water the plants real well again and that's it.

I usually treat the cage, go pressure wash a wall or 2 on the house, hit the cage again, do another wall or 2 and pressure wash the cage.

Pool cages are not so good to do with the roof cleaning method.
There's a time and place for everything.