Hi there. I've had outbreaks of Psoriasis on and off since I was 11, so 19 years (eeeek), so I know a fair bit about it

There are 2 types...one is stress related and one is a result of a streptacoccal (sp?) bacteria in your body.
My first outbreak which literally covered me head to toe at the age of 11 was stress related.....I was being bullied. I was admitted to hospital in the end for intensive UV treatment, as the creams etc were not working. Got a wicked suntan on the NHS

I've had literally every lotion/potion possible since then, and touching wood the last time I had it was when I was pregnant 5 years ago, although as you say, the scalp area is still a bit iffy. I have to admit that I veer away from the usual creams etc now as my skin is wasted, the slightest scratch scars now as the steroids creams have thinned my skin loads.
But, back to the original question....have you tried the 'Snake Oil' bar from Lush? We have one but as my hair is really thick I have never used it. Hubby has a very dry/flaky scalp and he swears by it, so may be worth looking in to?