Another update - THE BASS IS BACK! Arrived very early at 8am yesterday, and after a little time to warm up in the house and a couple of setup tweaks for relief and action, this bass is now 100% where I want it. Low, low action, fast to play, great tone, and looks amazing.
Robin has done a great job of getting the finish right - you have to look really closely to tell it's had work done to it at all - the neck binding in particular is a lovely piece of work, blending back into the original around the headstock.
Truss rod now works in the normal direction, with a seriously long key available (nice one!), the carbon rods are in there doing their job, and the whole bass just feels so much better. Thanks @Manton Customs!!
If anyone would like to see some pics of the completed article on here, let me know. Cost was seriously reasonable considering what's been done here - the bass overall cost to me is still a lot less than a new one, and I can have confidence that it won't need work like that again.
It really is a stunning instrument.