I don't like where that magnet is sitting.
Try a little experiment. Pop the magnet off, center it and move it a bit closer to the release spring. Hold it there with your finger, and then start the camera. Don't start the camera until you firmly have your finger pressed down over the magnet. Let the camera boot up and see if you get a percentage reading. If not, its most likely "dodgy soldering" as you've already indicated. If that's the case, try this--
1) Put the battery plate inside a cardboard box.
2) ship it back to where you got it
3) Buy this one that works. http://www.cinemaoxide.com/oxide-battery-plate.php