Hi, my parents dog, a cavalier, was on Arden Grange for a while and he suffered from a very itchy skin too.
May not have been linked, but they changed his diet and he is better now.
I use nutro for my two, and they love it and Lotties coat shines.
It's all natural ingredients without colours or preservatives.
It guarantees to improve skin and coat condition.
I think probably any of the more natural foods would help though, and I know lots of people here use Burns.
Goldie, my lab, was suffering from dry skin, but it has improved since I switched her to Nutro too, when we got Lottie.
It's worth a try.
I dont know about the evening primrose oil, was interested myself, so will wait for someone to reply to that too