Thanks for that. I successfully managed to at least locate a Line 6 folder that way, so I clicked on that Line 6 folder (inside VST3) and it's now listed under the add button (when clicking on that one), as pictured in the top half of the screen shot. - So at least Reaper presumably now at least knows there is a Line 6 folder present on the computer.
However it still doesn't seem to be aware of anything inside the Line 6 folder, as when I now actually try and load Plugins onto a Reaper track, selecting the VST3 section as pictured in the second (lower) screen shot doesn't reveal any Helix Native inside that, just the same plugins that were already there still appearing as the only options.
Apologies if I'm missing something obvious, - but any further directions/suggestions would be greatly appreciated.
Thanks again.