I ran 10 mags thru it today with the spring oriented as above (4 different mags were used, 2 with Lakeline billet plus one and 2 with Magguts). No premature slide lock back and 100% lock back on an empty mag. I'm not really sure what's been causing the problem but it's functioning fine now. If it acts up again I'll replace the spring.
All the help is greatly appreciated.
I guess this means Bawanna wins the prize.