Thanks for that....I am now making progress
I've corrected my import file to have columns for every possible choice, and leave the price column empty to choices that don't apply.
One final issue though, i'm getting my choices in a random order in the drop down box. ie some products are 14-16, 10-12, 18-20, others 18-20, 10-12, 14-16
It looks messy!