1 - the best material for striking pine cuttings is juvenile growth. In commercial P. radiata production stock plants are cut back regularly so they produce juvenile growth that can be used for cuttings.
2 - The tops I had cut off the seedlings were the ultimate in juvenine pine growth.
3 - The recognised method for making black pine seedling cuttings (cut the stem at the purple and set the top as a cutting) might not be the full story. It does work but the result is still only 1 plant from each seedling and

After tossing these thoughts around for a bit I decided to experiment and set 15 of the larger tips i had cut off the seedlings as cuttings.
4 weeks later and here's what happened: As you can see i got 14 rooted cuttings and 1 with callus but no roots yet.
Most of the cuttings have produced multiple roots around the base which is the desired outcome. to help improve the future nebari I cut back the new roots before potting up into 10 cm pots. as you can see this method is very successful and, as a bonus gives 2 plants for each seedling - the original seedling currently growing in its pot as well as a new cutting.
I still have a few seelings that I have not yet cut so I'll probably do a few more to see how time of year affects the results.