Hedley wrote:
Is there another approach to get a continuous rainbow of colours around the perimeter of the circle, and then do a radial blend from white in the centre to the perimeter?
Inkscape, or more specifically SVG, doesn't not support conical gradients. So you would have to simulate the effect which would be messy and complicated.
If it doesn't have to be vector then do it in GIMP. On your canvas, create a circle select and fill it with the "Full saturation sectrum" gradient using the shape, "conical (asym)". Then select a white to transparent gradient and fill the same circle with the "radial" shape selected. With the circle still selected, transform (or scale) it so it's an oval. Huzaah!
If you really really want to use Inkscape, you'll need to create a very slender triangle of a certain colour then clone it so each clone is rotated by the same angle of the of the triangle, group them then add a slight blur to them. I'm not sure exacatly how it'll turn out because the outer area of the circle may not blur enough whilst the inner area will probably blur too much.