I was recommended QuicSilver shampoo, cowboy magic yellow spot remover and gold label show white paste. I've just bought all three so I'll let you know how well they all work.
The only places i struggle with is tail and legs, i just cannot get legs clean, and i have to keep her tail plaited up so it doesnt get poo and wee on! Some great tips here though i will try some out. Im sure she is half hippo half horse which doesnt help matters!
Hibiscrub. If its good enough to get a clinical clean on wounds it is good enough to get the toughest dirt out. I find it works wonders on my bay horses white socks and blaze.