All done.
I fitted the new rear handbrake cable set today and everything is back to normal. There are no issues with the caliper, as per my initial diagnosis.
Incidentally, the aftermarket set of cables were well made with the exception of one thing. The pair of cylindrical lugs at the end of the cable were the wrong size and shape and didn't fit into my front handbrake cable's holder.
I had to grind off the excess metal in order for them to fit.
For future reference, in the event you need to change your handbrake cable on a GH model, you need to remove the near side under-tray that is located between the wheelbase of the car, but you only need to remove the bolts from the rear half of the off-side under-tray to get sufficient access.
If you have access to the Workshop manual it is easy, and my helpful tips for re-assembly are:
1. Unhook the release/rebound spring and re-attach this only after the cables are installed, the front and rear handbrake cables have been mated, hooked up and the adjustment nut is re-attached to the threaded screw.
2. Undo and remove the adjuster nut from the front handbrake assembly, and proceed to push the threaded bolt halfway down (1 inch) into the ratchet mechanism using a small screw driver. This will give you more cable to play with and simplifies the reinstatement process, where you need to connect the front cable to the rears beneath your arm-rest.
3. Once the handbrake cable lug is hooked into place behind the caliper, remove the screwdriver that was gently holding that threaded adjustment screw down, then wiggle the handbrake up and down and pull the cable back towards the handbrake lever. This encourages the threaded screw to resurface so you can attach the adjustment nut to the end,
4. My new adjustment nut sweet spot position is now 1cm from the top of the threaded screw - instead of at the bottom some 4cm away.
5. Don't forget to clean the grime off all of the handbrake cable contact surfaces behind each caliper with Plusgas or something similar.
6. Remember to re-locate the release/rebound spring before refitting your wheel
I performed tests when adjusting the handbrake adjustment nut to make sure that the 6th click of the ratchet would fully lock the rear wheels. Once the new cable is bedded in (one week should do), I'll adjust it again to make that the 5th click.