Hi Peter,
Two of my ferts went through the same itchy and scratchy thing, with hair
loss too.  I finally took them to the vet who prescribed a cortisone
shampoo that seemed to do the trick.  But was told to be very careful and
to make sure it was rinsed very well.  It has to stay on for 5 minutes,
which can be tough, but worth it.
 
I have heard not to bathe ferrets too much due to their sensitive skin
and then causing an overproduction of natural oil.  So when I do bathe
them I use a ferret shampoo by Tomlyn called Tearless Ferret Shampoo,
it's supposed to be baby mild for sensitive skin.  I bought it at the
Ferret Store on-line, I think.  They usually only get bathed when they
have gotten into something, like when Mokey dived head first into the
duck soup for some reason and had to roll around!!??  Who knows why~!
 
Good luck,
suzi
P.S my pom-doggy has sensitive skin and I use a oatmeal based shampoo on
her that works well.
[Posted in FML issue 3964]