It was a mixture of things. Timing be out, dying coil packs and dying ptu.
Then the non starting was down to a blown fuse.
Tbh I was at my witty end with the car. I even went out and got another good running gto just to use as spares as I thought maybe the main ecu was kaput.
If still no good in a couple of weeks, I’ll get up to quinton again in me gto and if needs be you can try my ecu
As @Mesobitchi timming could be out!!
I recently changed every singke thing and it did nit make no diffrence …eventually the timming was out !
Chngaged the hydrolic tentioner and it was all perfectly fine !
So i would recommend to check the timming
Check the link below the issue i used to have
@Mesobitchi yeh mate that sounds like an idea add me on facebook n take my number , giz a shout when ur about mukka, im matt shadowsniper wood on facebook, 07927256701 - thats ma number
My money is on timing too. You say it went bad overnight but chances are it went as you were cranking it.
Have you put fuel in it recently? Take a look at the inside of the tank as someone on here hadn’t started their motor for a long time, was ok for a journey then had similar problems to you. The tank was all rusty inside so the complete fuel system was cleaned and a second hand, clean tank was installed and now runs well. Can’t remember who it was tho.
Terry 
Mine was doing that I changed the fuel pump relay sorted it out