When I bought the bass, the treble pickup was not a genuine Ric - whatever had happened to the bass in the past, someone had fitted a fender Jazz style pickup onto the Ric baseplate. So I got a genuine Ric treble pickup complete with surround and cover from ebay. I carefully examined the wiring from the pickup when I bought it, and it all looks original, right down to the heatshrink wrap over the soldered joints.

I really am quite baffled as to why the pickup is behaving in this manner - so any advice is most welcome.