I sent this drawing to a spring manufacturer in the UK and he sent me 2 nice St St Samples which are working perfectly. To buy them, there is a minimum order of 500-1000 (from memory), and I was not prepared to put up that amount of money in advance.
This spring came from a K0 advancer. My view was that if the wire was a very similar diameter and the turns were the same along with the other dimensions, then the resulting force it will apply will also be within a very good tollerance of the original spec.
As I say they are still working well
[attachment=0:125ui292]Advance spring for 450 Model (1).pdf[/attachment:125ui292]