spray them with this 
http://www.thecleaningwarehouse.co.uk/clover-viro-sol-citrus-based-cleanerdegreaser-169-p.asp leave for about 3 min and just brush off they come up a treat. i have done some today and this stuff is fab going to order my self another 5x5lts soon.
Hi Dave,
I got TFR - waste of money.
I got Ubik 2000 - waste of money.
Both products were highly recommended by people on here. The problem with Ubik 2000 is that it doesn't clean stains on plastic at all; yes it removes green moss and any grease but that is it. I always revert to cleaning the plastic with Cif on my brush and scrubbing like mad. I hate conservatory cleans as they take me too long. I made the mistake of leaving the TFR too long the first time I used it and spent 8 hours cleaning the conservatory roof and sides by hand getting the TFR stains off. (Infact on the neighbours side of this conservatory there are still TFR stains visible 5 years on which I didn't get off completely)
I see that Viro-sol is made by the same company Clover as Ubik 2000 and both claim the same results, but my experience is that Ubik 2000 doesn't do what it says it does on the tin.
Does Viro-sol bring up the white on UPVC conservatory roofs with minimal effort?
We used TFR in the valleting bay at the garage in a steam cleaner which worked well for getting the wax protector coat off a new vehicle paint work before delivery to the customer.
On Peter's forum there was a cleaner who uses thin bleach. His customers are delighted with the results and he uses minimum effort. I'm scared of bleach and haven't tried it as it can kill plants and fish if it's residue gets into a fish pond. Another thing with bleach is that it doesn't clean off the stains, but hides them by making them invisible, and I don't know what to think of that. It also takes the colour of of the Bib and Brace overalls I use.
Spruce