Aye Meddle, I've had a wee look at the 50s les paul stuff on the net, one guy summed it up as "Exactly. That's why the 50s wiring is 'wrong' from an electronics point of view, because it makes the two controls interactive - when both are below full, changing [i]either[/i] affects the setting of the other." which is pretty much the issue I had.
Well worth getting changed on this bass IMO.