franzbassist

Duff string from the sounds of things, particularly if it is like for like.  What string is it?

Have you tried pressing it down firmly either side of the nut and the saddles and then trying to intonate? 

Thanks guys.

It's a taper core La Bella white nylon, which have been my standard string on all basses for ages.

Yes, tried the witness point trick, taken it off and refitted it, nut slot and zero fret checked.  

Very frustrating!