...you mean you can't actually become an expert armourer until your fighters upgrade into either mercenaries or...rangers or whatever?Okay, that makes sense, but I saw nothing about it in the manual, and they could have made it a little clearer in the game.
I guess this means that certain skills (like bow) can be used by fighters but other skills, like armourer, you have to be a better class of character altogether?
In that case I guess what I have to do is figure out how to promote one of my characters from a fighter to something better. Thanks for the help fiffa, much appreciated 
