The mkIV has normally got a ballasted coil feed this is a resistance wire in the loom , its a dull white pink colour this feeds the coil and your electronice unit ,, from the ign key switch the wire drops the coil voltage to around 6 volts, its not easy to check but if you earth the coil negative then check the readings
it may be the low voltage is not liked by the dizzy electronics , some wont work below 8volts
there is a second wire from the starter solenoid that by passes the ballast wire when you crank the starter, this puts 12v direct to the coil to aid starting in poor battery condition.
to get the car to start both these circuits need to work
if you have a meter check the ohms across the coil blade terminals
if its 3 ohm you need a 12v feed all the time
if its 1.5 ohms it must have the dropper wire in use and make up a 12v feed for the dizzy electronics
if its got a 3 ohm coil and has been ballast bypassed to 12v this is the easiest option as the dizzy electronics will have a 12v feed , not a 6 volt feed which can be too low.
Not sure tbat makes any sense
Pete