I've tried to get round this by changing it to:
SELECT DATE_ADD('2015-01-01 19:30', interval -1 := -1+1 week) AS start_date,
DATE_ADD( ADDTIME('2015-01-01 19:30', '01:30' ), interval -1 week) AS end_date,
num.* FROM num
WHERE num.i IS NOT NULL
Perhaps unsurprisingly (as I don't know what I'm doing!) this hasn't worked. I don't really understand why it works in navicat as a quert, but doesn't work in Dreamweaver.