Hello wickedtart, I am a Decorator, if the artex ceiling has a stippled finish it shouldn't be to difficult to repair it, you could try plaster or something like polifilla, you would mix it with water so that it is so that is about is as creamy as slightly runny porridge,apply is thickly to the damaged area and then use a cheap small (about 4x3 inches) stippler available from diy or decorators merchants, you dab (ie. stipple) the wet plaster with the tool , if it is too spickey or flat scrape it off and mix a bit more,either thicker or thinner and repeatwhen totally dry repaint the ceiling or try to touch up just the repaied area, best of lick, Pete.