Hugh is correct, the bass does indeed drive headphones directly - and really very loudly as well!
The two pickups are wired in stereo, left and right. There is no discernible stereo effect on headphones, it's not as if one pickup is closer to the E and one to the G and there is a spread across the image. However shorting one pickup clearly removes the signal from one channel. (I'll post a photograph of the very Heath-Robinson "test environment" later on :D )
Testing the pots more comprehensively:
A - Volume
- R15= 1.004K
- R13= 1.004k/5.9ohm
- R35= 5.9ohm/1.004K
all makes sense, operating as pair of potentiometers with 3, 4 as the wipers.
makes sense with 1+3 / 2+4 connected together, and then operating just as a variable resistor to change the tone.
So perhaps it's not the pot that is knackered after all? What's next to look at?
As ever - huge thanks to everyone.