Instead of wrapping wire around the plug lead, I have seen a small bulldog clip like this used as a capacitive pickup.

 

If you really want a continuous LED indication, then you could add a diode and capacitor to the gate of the MOSFET circuit suggested by MisterBill2 (example circuit below):

View attachment 342859

Finally got the circuit working.
As is it would not work, after much experimenting i added a 1N4148 diode from the signal wire to ground. The LED now lights with the wire just laying next to the HT lead. There is flicker at low RPM but isnt an issue, grabs your attention!