anyone know what the regs are on position of number plates?
Yep. Needs to be straight and on the outside, and if your car has a plinth it needs to go on it. If you put a aftermarket bumper/us style grill on, and your plinth vanishes, still needs to be mounted on the plinth location as the changes are entirely self made. Luckily knowledge of this part of the SVA regs are not well know and you can pretty much put it anywhere. Dash is a no no as it's not straight, nor outside. No case law has been created on this so no official direction unfortunately! As long as you don't take the pi55 you wont be botherd. Mine has had the 'look', but nothing more.
Mine is currently sitting on the bottom part of the lower grill as lovingly seen in the photo below taken on Friday.