Afraid there's no lateral support along the bottom, only 2x at the top... Tank is now up and running. It's not entirely level, about 3mm difference from one side to the other (based on the distance from the top of the water to the top of the tank as it barely affects my spirit level), but I'm comfortable with this, given the additional bracing around the bottom of the tank, and the relatively tiny difference in level.
It's now all up and running... looks good. Plumped for a sand substrate for the corys, and fake plants for ease of cleaning. Nice chunk of bogwood in there too, the slight staining of the water helps to make all the plastic plants look more natural by dulling their colours.
Thanks to all for their help/input, you guys have been awesome!