thanks for all the details Rob and i think we can narrow it down to the hook knot,
a quick search shows me a few variants to the Grinner knot. if you are indeed using the Duncan knot (he invented it but was stolen, published and re-named 'Uni - knot') http://thelimpcobra.com/2011/11/15/the-duncan-knot/
which is basically the only hook knot, leader to leader, and it's variant as leader butt to line that i ever use and fwiw, fully endorse.
there's only one thing i can think of that a lot of personal testing and use has shown that might help your problem.
you mention making five turns, this might be too many.
i can't explain why but contrary to just about every other knot the Duncan doesn't like any more than 4 turns max even with 0,8 or 0,10 mm mono. any more and it weakens it.
any mono bigger than 0,20mm gets 3 turns and over 0,40mm only two.
it's worth experimenting with, i hope this helps