Echo the Champions Tail products... I use them quite a lot and they seem to be rather good. I do use the concentrated stain remover on his legs prior to shampooing though and it seems to bring him up quite well.
Have used the Oster shampoo for greys also and thats not bad (it also smells divine... vanilla and something..)
Just a warning about washing up liquid. Yes, ok to use on tails..... be very careful using it on the body. Grey's have very sensitive skin and I stupidly followed the advise of someone who told me to try washing up liquid - my horse was covered in an awful reaction the day after which quickly developed into something very similar to odemas.
Think the best thing with greys, as others have said, is to be prepared to wash them twice. Once with a tough general shampoo and a second time with a whitening shampoo. Use a water brush on their legs... rubber curry and sponge on their bodies and remember warm water! Cold water wont get stains out.