stalled at petrol station, wont start

So i was on my way to work. popped in to station for a paper, left it running. Set off and it died. wouldn't start.

Got towed to work by a colleague and had a look under the hood, nothing obvious and with limited tools Ive held the HT from the Coil/Balast near the engine block to see if it sparks and i don't get anything. Not sure if thats the right way or i need to do it with a spark plug or a spark plug lead rather than the coil/ballast?

Voltage to the coil/ballst drops from 12v to 4v under crank. tested across the coil terminals

Was going to order one but figured id check incase I'm testing wrong
Any ideas?

Ta

Sounds likely. If no HT sparking, next step in rave is check voltage of battery. Then check coil POS to battery neg and coil neg to battery neg, reading 1 volt Max below battery voltage. Ignition amp screw to ground 0 - 0.1 volts.
Assuming you have electronic ignition. Test with ign on, not cranking

yep electronic ignition.

Will test and post back.

Ok tested and got the following.

Battery 12.24v
Coil + to battery - 12.09v
Coil - to battery - 2.3v
ign amp screw to ground 0.0v. Presuming you mean the module on the side of the dizzy where the coil wires go to

Remove Lt lead at coil neg and check again. If voltage still incorrect, fit new coil. If voltage is correct, check Lt lead. If Lt lead OK, fit new amp

tested as you described with voltage incorrect so got a Lucas could for £18 which fixed the problem. Much appreciated.