someone on here recommended Caravan Cleaner from Aldi. I got two 1 litre bottles £2.99ea or some thing like that. Used it for the first time on a green conny roof and it did a great job. Diluted it and brushed on with an old brush then after a few minuted brushed of using tap water from outside tap. Didn't need to use the pure water, finish was excellent. I think you would get five or so conny roofs out of each bottle, so pretty good value.