Thank you for your replies.
Her cystitis is idiopathic and to be honest after 16 months of getting this, I've come to the conclusion that Cystease is the one thing that really helps her. I did change her diet over to 100% wet cat food, although I sometimes put a few crunchies on top of her food with the cystease in it if she isn't interested (she has a big fat brother who will eat anything in sight, so you can't leave food hanging around for too long). Tab, my cat is the opposite, Feliway doesn't seem to make any difference to her, although I did have one plugged in over Christmas with so much going on and for holiday times.