This is what was doing my nut in - the bass is shielded really well, and for a while my thoughts run away with me.
Someone had added a series/parallel switch to the bass, and I never use it, so I put it back to stock just in case there was an issue with how it was done, even though it looked fine… but no, noise still there.
Then my brain kicked in, and when checking the chain I used the blend knob on my compressor, and yep, no naughty noise when the blend was dry (or when taking it out the chain), but it came in when going wet. I guess the comp was amplifying the fact the pups are single coil, and picking up interference from the room’s electrics perhaps.
Recorded the part where the noise was previously an issue, and the bass sounded fine (until I decided my Stingray was actually better for the track after all 😂).