I've used the Screwfix stuff on a few conny roofs now and seems pretty good.
One from today:
What dilution rate are using Andy?
I find 1 litre of screwfix into a full backpack works best.
Also it works best applying it with a brush with fan jets, give the roof a quick go over, gently agitating the dirt whilst letting the chemical soak in.
Leave for 5 mins then wash with pure.