LizG - I have had psoriasis since i was about 17. A few years ago it became much worse as I had a reaction of drug treatment. I found advice from dermatology was brilliant. The advice was always to moisturise, even when the psoriasis was in abeyance. I was prescribed Hydromol Cream and ointment. I had hydromol bath/shower wash.
The advice regarding steroids was to put moisturiser onto clean skin. 10 minutes later, put on the steroid cream. I had various strengths of steroid cream over a period of months, because the flare up worsened initially, rather than responding well. The most useful advice was not to stop using the steroids as soon as the skin began to improve. If you just stop, a flare up is likely. Go from twice a day, to once a day, then to alternate days, then every 3 days, over 2 or 3 weeks, until you use the steroids twice a week, then once, then stop. Never stop moisturising.
Good luck with it - it's horrible isn't it.