The mystery deepens! Having now fitted another new coil, the non-starting continues! A meter across the + and - connections on the coil gives a constant 12 volts with the ignition on. A spark tester fitted to the king lead produces a strange result - on cranking the engine there are two initial good sparks and then nothing, but, when you stop cranking, there is a final good spark as the ignition / cranking is turned to off. I've substituted the original ignition control module for the new one and still get the same result, two good sparks then nothing no matter how long you crank the engine, and then, on turning off the ignition, a final, single, good spark. I can't find any mention of a ballast resistor or a capacitor in the Ford Mustang workshop manual and neither have we found any sign of a diagnostics port. There is an engine run relay in the central fuse box behind the dash but substitution doesn't make any difference. The engine management IC is new but replacing the old one makes no difference - the symptoms remain the same. All fuses are good (last resort stuff now!) but I'm running out of ideas. Could it possibly be the ignition switch itself? All ignition type wiring and plugs look sound and all contacts appear good. Thank to everyone for their ideas so far! The original alarm system (FoxGuard) was replaced about 7 or 8 years ago with a Toad alarm. Any ideas on whether that could be the problem? I can't even tear my hair out because I haven't got any.......