Ok, we're cooking on gas again, or should that be induction these days?
Thanks @Hellzero for the diagrams. Mine didn't resemble either layout exactly. My tone cap was only on one pot, like the later layout, but I didn't have a dedication ground across the back of both pots. I've rewired as per the 73-95 layout and added a ground between the pots. I also sucked all the old solder off and redid all the connections, trimming leads where required. Whilst this bass has been worked on over the years, the pots, cap and jack socket are the originals ones from '73.
I do suspect the issue I was having with it was related to grounding, as it sounded like that, and one pot was every so slightly loose (now tightened). All working well now, and quietly, and the tone isn't any different than it was before, which I'm very glad about, because this bass sounds really good.
Cheers,
Rob