global herbs Footstamp (Pestoban) has been great for ours over the years.
Also been recommended by a vet to use an anti-dandruff shampoo thats got Selenium Sulphide in it - i got some from boots over the counter called Selsun. Recommended to use that twice in one week then every other week, it kills the mites. I have used it once and i must admit she hasnt scratched her feathers since!
If persistent itching even after washing quite a few times then get the vet for a dectomax injection, always works for ours.