Looks like Williamson's credibility (not that he had much) is undermined from out of his own mouth.
Again from The Guardian - he claims that 'We don’t get any detailed data before schools but when we started to see concerning outliers... that’s why I felt action had to be taken.” and it has been noted this means that he announced his 'triple lock' before seeing any data outcomes: so either he is utterly useless in not checking before announcing, or he is lying (or, of course, both).
I'd imagine, as the say in these parts, that his coat is on a shoogly peg.
