I think that it is difficult for the vets to diagnose what is wrong with out little friends. I am not sure whether they just start with the most probably diagnosis and then eliminate ailments until they hit the right one.
I am out of my depth with skin diseases and don't know how easy/hard they are to differentiate. If you feel that the vet was negligent and should have made the diagnosis complain and see what happens.
But if the vet just didn't have the right information or symptoms to make the right diagnosis that is just down to being human.
Try bouncing it off
https://www.facebook.com/PeteTheVet and see if he can give you an idea of how easy/hard skin diseases such as this are to diagnose. He also has a weekly problem page in the Telegraph.
I hope Alfie is feeling better soon.