Okay everhone. Update time.
There is a complete, 100% fix that sorts the issue completely that I've found. If I tune the bass down a half step to Bb the issue goes away. It's not a solution for me as I'd go raving mad trying to work out what I'm doing, but it's good to know it's an option.
This is what Ryan at Dingwall had to say:
Thanks for reaching out. In my experience dead spots can be minimized but not necessarily eliminated without, in my opinion, unacceptable compromise. To remove them completely the resonant frequency of the neck needs to be octaves above that of the speaking range of the instrument.
Full carbon fibre necks work great for this but the tone for some people can be a deal breaker. We also had great luck using a clamp at the headstock. The added mass from the clamp helped a bit for sure. The major difference came from the off-centredness of the clamp, this all but eliminated the dead spots by controlling the torsional vibrations in the neck.
I've heard some people have had success over the years with the Fatfinger, but if adding mass to the headstock is the goal the first thing I would try is a heavier set of tuners.
I hope this helps, please let me know if you have any more questions.
I asked what tuners he'd say worked and he told me "Gotoh's are a drop in, the index screws are located in the same place. Look at the standard keys, not the reso-lites."
However... on recommendation from someone else I had a go at adjusting the truss rod. I'd done it as soon as I noticed the dead spot but only gave it a quarter turn so this morning I tried giving it another quarter turn which is about as much as I want for the action I like (there is still a bit of relief in the neck) and while it's not totally elimiated the dead spot, now if you didn't know where it was and I handed it to you I'd challenge anyone other than a luthier to spot it.
So, that's it. It'll do me and I'm not doing anything else for the time being. I'll see how it goes when I change the strings next or if the neck needs adjustment as the seasons change or whatnot.
Thanks to everyone for your input - I really appreciate your advice and support.