My springs are mounted on 12mm bolts, but worth noting that mine are not under the rails but to one side, the the bolt is not supported on both sides.

Ref locating axle on the spring, As an alternative to drilling, what about gluing (or wrap and glue) a longitudinal locator in place?  It needn't be a pin but more of a saddle/perch?

I used 10mm bolts on the front of the front springs and 12 on the back. The rear leafs are 12 all round. They all come out of the side of the chassis.

I would agree with that Chris, providing that the spring doesn't already have the centre bolt hole.
There has been an instance of a leaf snapping inside the clamp plates which did not match the spring curvature. Break was at the hole, and it happened while being transported on the road to Stretton.

Ive been working on a picture, which has helped in some ways to get some details in my head, but also given me another couple of ideas to think about, mostly to do with sensible ways of mating the wooden and metal parts without causing problems in the future.. But basically semi elliptic and hung outside the chassis rails. I’ve been reading a load of stuff about shear strengths and tensile strengths of different sizes and grade of bolt, I think M10 will be enough. I watched some videos (Americans) where they used 3/8” bolts (9.5mm) so M10 high tensile should be fine.

Heres a sketch of the side view of what im going to do- basically a shoe that will bolt over the ends of the chassis rails (that will get trimmed down) and then all the bits hang off that. The inspiration cars chassis is so arranged that the springs sit outside at the front and inside at the rear- im going to have mine hanging down inline with the rails at the front but inside at the rear. My chassis just isnt kinked in as much as the real thing at the front end.
Im going to use some weld on hinge straps designed for 1/2 inch pins as eyes at the front of my springs, suitably trimmed down to size. Ill fabricate something to mate with the shackle at the back out of some steel stock.