A well respected US contact has upgraded my drawing ...

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He assures me this WITHOUT a pre-amp will work well, two volume pots to blend peizo and mini humbucker. He said being a resonator and not a smooth sounding high quality acoustic the lack of pre-amp is fine.
If a small affordable pre-amp turns up before I wire it I'll fit one (and can retro fit later) but for now I'll set the neck and work with the attached plan in mind.