Got a reply back from the NZ transformer manufacture.

..."We anneal our GOSS cores at 800°C. We control the temperature up, soak, then control the temperature down so there is no thermo-shock.

We haven't had particularly good results re-annealing cores; they do not generally improve performance (from a current transformer accuracy perspective)."

He recommended demagnetization the core of any remnant flux from previous usage; its easy to do according to Dr Google. Also check the surface faces for micro burrs that maybe shorting the laminations.

Mike