I have done a lot of experimentation, and the only logical method is as follows:
<g transform="translate(0,400)">
<g transform="scale(1,-1)">
Where 400 is the height of the image. What this does it move everything down so that the top of the image is now and the bottom of the image, then the scale operation flips the Y coordinates, so that the bit that is now off the page/image is flipped back up to fill the space left behind.