Update.

Have played with the adjustment of both the lever and the cable mounting on the gbox.
Upon testing, you can feel the gearbox/transfer case actually pulling itself out of gear and into neutral.

Stopped through Japline on my way through today (in their new workshop now) and Mick mentioned that he had seen a few of the dual range boxes with a similar issue. A common 'fix' for them was a bit of wire (think of a coat hanger) had been used to keep the mechanism from flicking into neutral. This may see me get by in the short run, but the wear of the synchros/bearings in the box will only get worse.

Will try the patch up for now, and have my eyes open for any cheap gearboxes (preferably not dual range lol) that will mate up to my existing setup.