I scrub
them off once a year (birds...).

Hi Martin
I also have the CloudDancer covers and got a bit of mildew on them when I put them away damp one year. Here in the US there is a product called "OxyClean" and I ran the covers through the washing machine with it and it removed all soil, mildew and restored the bright white color. Try a small component of the covers (like the tailplane cover) in the machine first.
ROY