There's no satisfactory answer to this one. Anything really effective at cleaning grimy terylene sails is also likely to damage them. Usual DIY treatment is put them in a warm soapy bath to soak and get rid of all salt, then swish them round and a bit of hand wringing to remove all the surface dirt, rinse, dry thoroughly and store. Even if you have them professionally valetted, they won't come up much better.
BIOTEX. Tried it last year following a Forum recommendation. Works by soaking the sail (suitable size conatainer req'd!) for 12 hours or more followed by thorough rinsing. Gets rid of greyness and grime (but not metallic or tar stains). Bought at some supermarkets and hardware stores. Considered as safe for terylene.