The car's getting close to being finished now and I'm starting to think about the SVA prep work. Am I right in thinking if I get a bit of wood, steel or whatever and drill a 5mm hole in it then lop it in half that will be similar to what the SVA man will use to test the radius of sticky-outy bits?

I take it if the edges are resting on something then nothing must be able to touch the inner of the semi-circle?

drill a 5mm hole then cut out a corner of the piece so that the straight edges go to the edge of the circle. You then have an L shaped piece with a 2.5mm internal radius.

That is one decent way to make a rad gauge. You may also find a set for not too much money at an tool suppliers.

In the past ive tried to measure them with a set of verniers. Not terribly accurate if the rad is borderline but gives you an idea how close you are to the spec.

That's the usual way of knocking one up, however standard radius gauges are a quarter of a circle, not a half. If you use a semi-circle you won't actually be able to get it onto most radii to measure them.

Or you could buy one of these for £8.32

eBay link to set of radius gauges