Were they ok together before Kirst? Did you see how it started and was there anything leading up to it that might have sparked it off? I'm thinking something that Fredcat found unpleasant - a loud noise, seeing a strange cat outside, anything out of the ordinary? that he might associate with Stevie? (often it can happen that the attacked cat just happened to be passing by and is associated with whatever the attacking cat found unpleasant). Having said that, it can just happen out of the blue, I remember it did with Harry and Jasper - and I was there, and there was nothing that I could tell, had set them off.
You could try the scent mixing thing - you know, when you rub a cloth around each of their heads (mouth and cheek area) like when you are introducing a new cat.
