quite common on fiestas and escorts to have dodgy wiring connections on the starter and once disturbed start to cause problems! peel a bit of insulation back off the main starter wire (the thick one) and see what the wire looks like, these can go very brittle and give conductivity problems if that is ok get hold of a test lamp (one of those probes that you earth to battery and they light up when the other end is pressed to live) and hold it on the small exciter wire while someone attempts to start if it does not light the test lamp then there is a problem on that wire