Bee had a really scurfy tail this summer and quite by chance my hubbie came home from caravan deal with a bottle of this http://fenwicks.info/Horse_Shampoo.htm as a gift from the owner who knew we had horses. It was brilliant stuff, there is tea tree oil in it which I think may have done the trick. Think it retails at £10 a bottle but it's very thick stuff!!
feed Feedmark's Equidermis - i use it on my mare as she used to have a scurfy mane and tail, now they are always clean and scurf free - makes her coat look fab too. If you put products on it it just makes it look greasy and horrible.