In reply to GrahamD:
I've just been into the garden, set it up and jumped out of the tree onto a 8.5mm dynamic rope.
What I found:
1. using two half hitches works fine, but is harder to undo once loaded.
2. the second half hitch is critical and hence you need to create a long enough bight to tie it. With just one half hitch I was able to get it to slide to failure by just pulling with my hand. Interestingly, which strand you pull makes all the difference: one slides, the other tightens the knot.
3. Very little load is passed to the second half hitch, which means the first one is tightened down and gets very small and hence moves away from the second - which is then left flopping around and might well work undone.
4. However, the first one is now so tight that it does not seem to be able to slip like it did under point 2.
Note, these were low fall factor, low energy falls. If someone can pull 12kN and check if the above is still true it would be great.