I had this problem with my 93 TD. I fitted a replacement starter motor.No joy. Dirt builds up in the ignition barrel, this causes a resistance and drop in voltage to the starter(only about 0.3/ 0.5 volts) This is enough for it not to turn over. Remove the ignition barrel and spray wd40 through it and see the dirt that comes out. My ZXstarted first time and has run ever since.
Rob, March 2009