Lost of lights and engine cuts out. HELP!!!!

Hi, finally the mgtf my friend asked for help with is ready to roll. However, It has started and run over the last few days without a problem. So On its maiden test drive this evening it seemed a bit reluctant to start but the fourth turn chugged into action. Alls well we pulled off turned lights on and drove about 1/4 of a mile when without any indication all the lights went out and the engine stopped same time. When you turn on the ignition nothing at all. we pushed it back to a safe spot and after 5 or 10 mins turned the key and it started again. Driving it back the short distance it seemed to behave as it should.

So the subframe and engine have been removed and as far as I could see all connections are in place and the earth strap at the back by the large fuse 80Amp I think.
Any ideas I sort of feel I'm helping out for the long haul on this project and auto electrics really isn't my bag.

Ricky

How well charged is the battery and how well is the alternator recharging it?

Is the earth attached to the diff case?