I then tried something a bit more reasonable, a 20x20x20mm test cube I knocked up in CAD, after setting the Z axis properly, and it came out great!
I've tweaked the contrast a little so you can see the hemispheres I added for the numbering, turning it into a d6. Accuracy of the print is amazingly good, it is +/-0.12mm on all the axes! (I printed it on fine - 0.20 layers) It's not perfectly cube shaped due to not being orthogonal, it's about half a mm out if you measure the opposite corners, though it's not easy to repeatably measure corner to corner to corner like that. It's certainly cube shaped enough for you not to notice anything wrong with it.
Defo got to move it out to the garage, though, as the fumes are a bit nasty.
Fixed it now, I've stuck it on my blog!