Righto I think it's nute tox - specifically I think what happened was an accidental big overdose of CalMag raising the EC too high. It came on way too fast to be a simple deficiency or lock out, and the PH wasn't so far off as to cause major issues. I've treble checked and the temp hasn't exceeded 26 degrees, nor dropped below 17. I flushed her through with PH water today approx 40 litres - until runoff was clear and correct PH. I am not going to nute her for a day or two.