I don't buy Benz or any other "branded" chemical - sodium hypochlorite is readily available from pool suppliers and other cleaning outlets ( those who deal with farmers and the like ) the cost is far less - I pay £15 inc vat for 20 litre drum
I don't use it on plastics such as conservatories as a rule, however I have used it as a mild solution for one off stubborn areas as you describe approx 1 part hypo 5 parts water through backpack at that distance no need for nuclear fallout suits
- if spraying then wear a waterproof coat leave on for 15 mins and go algae should remain.
I have to say I'm sometimes tempted to do sfg cleans this way, but I know you can't really beat a proper scrub with a brush and chem like virosol/TFR etc.
Darran