Thanks dario and zwicky.
The aim was to see if I could squeeze in a loop line inside the second radius with enough space for an island platform, and without decresing the curve below 2nd radius. I have pasted a diagram below.
http://i8.photobucket.com/albums/a27...unk/Layout.jpg
Having now got all the bits and spent all day with bits of cotton and drawing a spider's web of arcs on the board,
the answer to my question is 61mm (+/- 2mm) Incidentally one factor is that Hornby have used
straight point blades rather than curved, which gives a slight flattening from a true circle.
I have used the "springy bit of piano wire technique" to let the new track loop find its own geometry (aka cubic-spline) rather than trying to set out elipses with two nails and a loop of string (
almost y = x(squared) curves)
This isn't some fancy exhibition layout by the way. Just some old plywood and track for the kids to play with.