Erm....and for IanC who is viewing through oddly coloured spectacles, I said 'which' O-Rings when you asked me if I had fitted them on the 550s, meaning that there are a number of different ones on each injector.
I then picked up an old injector to find the one on the nozzle was missing. Then found this to be the case on them all. At that point I needed no help at all to come to the conclusion that after fitting new 550s with o-rings, there had to be 2 in each hole! If I didn't know about the O-rings that were meant to be there, I wouldn't have fitted the new ones to my new injectors and ended up with two in each hole.
The problem was that as I couldn't see into more than one injector hole, I couldn't see that when the old injectors came out, they left the nozze O-rings in the holes.
Sorry, mate, just couldn't let you get away with that one in public.