Thanks for help so far! What we've got so far is still factory wiring, just with the replacement pickup and 1 250K mini pot audio taper. I thought i'd keep it at that to begin. However, none of the wiring diagrams i've found line up exactly how this is wired.
Thank you for the welcome - I am indeed new! I've wired a fair few guitars up before, replacing components, mods etc but never done anything with bass electrics. Getting inside this one, it looks like a very unusual way to wire up pots and pups but maybe not!
I tried inverting the phase on the pickups again. Result: Precision massively quieter than the jazz. So how I had it originally was closer where both pups would work standalone. I also tried switching the cap so the leg went to the outer of the pot, rather than the central lug (obviously swapped the signal wire to the other pot too). Result: Precision worked great, no jazz signal at all.
I've wired it all back to how it was in my first message. Rather than faff with pictures i've sketched up a crude wiring diagram. Compared to [url="http://members.ziggo.nl/s.k.wiersma/images_bass_building/Wiring%20diagram%20-%20std_p_j_bass.jpg"]this diagram[/url], I can see some differences but to my eye nothing massively wrong...however, i'm so far from a professional it's laughable!
Anyway, I'll cut to the chase - thought i'd throw the two diagrams on here and see if you guys see anything! Next plan is to rip apart all existing wiring and start a fresh with new wiring (getting rid of that nasty multicore thing thats in there too!)
John, I'm curious about your drop-in harnesses - you may well be hearing from me depending on how this goes and yes, Lindstrom tools! Both my Mom and Uncle we're at BT circa 1970~1980 and I believe they were standard issue. We've a fair few pairs of pliers and cutters around and they are really brilliant, even after all this time!