Its cheaper when they sell it on Ideal World on the tv Belinda as you some times get a bottle for free..check out their site..sells in car shops around £17 per bottle but,lasts for ages and you dont need to wash the car/caravan before you use it though,i would wash them if applying for the first time.Its a waterless cleaner polish and is fantastic...got me started now
The best cure is preventing it in the first place
You've got to clean the roof that's where they come from
Get up on steps and clean and rinse the roof twice before you start on the body
Then spray it all with Bobby Dazzler,roof too
I wash my van about 3 times a year this way and never see many black streaks forming if any at all
Just keep that Roof clean