Finally - found the bluddy culprit !! The middle white block of the 3 in the preamp , below , was the perpetrator, and nothing to do with faulty blend pot or faulty pickups.
I had the bass on the floor, vol up, and dialling the blend between pickups, whilst probing the various terminations. Found that middle white block had some dodgy crackles, and tapping it cut the volume and tapping again brought volume back. Pulled control cover off, fired up the solder station, ripped off the sticky back insulator, and heated all the solder joints up till i see the solder flow.
Job done - everything is as it should be. @tauzero Think you called it. Dodgy preamp, but only a slightly failing solder joint rather than failing preamp. Which is what @Marco.EB advised. So you were both on the money.
Which joint, dont know, cos i heated them all up, but carpet bombing all joints has sorted it