OK, panic over. Look back at some of my first threads and you'll see the answer!!
To finish troubleshooting what was happening I hooked up the PC to the car battery. Guess what??? It all worked better than hunky dory!! It would appear that even though the desktop PSU was rated at 300W it may not have given the correct amperage (or something) to kick the M2 back into life. Tried it in the car. Went into hibernate. Left it for an hour and connected the J3 back onto the battery and it powered up straight away.
Woo-hoo. Now to fit properly in the car.