Welcome Dangermouse! They're quite a few of us around who don't have the skin disease, me for one. I was lucky to be diagnosed fairly quickly, but I had a very good idea of what was wrong courtesy of a medical family background.
If I were starting treatment all over again what advice I would give myself with the benefit of hindsight
1 Get to know your rheumatology nurse at the hospital well. Ring up and introduce yourself if you haven't met her/him - valuable resource especially since GPs are remarkably ignorant on psoriatic arthritis is no doubt you've discovered!
2 Make notes of the questions you want to ask for a consultant's appointment. Start making the list as soon as you have questions, even if it is after the last appointment... most doctors appreciate it if you have taken the time to sit and think about what you need in the way of information.
3 Expect to be your own personal guinea pig when it comes to finding the right drugs for you with a process of trial and error. Sometimes it's so slow that it's very difficult to keep track of. I didn't think the methotrexate was helping me until I had to come off it for antibiotics. MMM instructive lesson number 1. Patience is needed because of the drugs take a while to do their work, which makes you want to howl with impatience.
4. Find an aqua fit class to keep your muscles and tendons moving without stress on them too much. I think one of the problems of losing mobility, is that it can lead to feeling low and ultimately depression. you will feel a lot better if you can keep some form of exercise going... even if its from a chair!
Anyone else got anything to add?