I think I've got it right but not 100% sure how radius test works?. So I've drilled a 5mm hole and cut it in half for a 2.5mm radius, I'm guessing the aim is to have curves round enough to not be able to touch the middle of the curve?

Getting close to finishing now and starting to flap about every edge on the car, specifically the chassis tubes which aren't covered in passenger compartment

You want a quadrant of the hole to check a radius.

http://en.wikipedia.org/wiki/File:RadiusGauges2.jpg

Yep, cut your template again so you have a quarter of it and then use that. As you say, the edges shouldn't touch the middle of the quarter.