A 3-day-old term infant, exclusively breastfed, develops jaundice. Total serum bilirubin (TSB) is 280 micromol/L. The baby is otherwise well, feeding adequately, and has no signs of sepsis or haemolysis (normal FBC, negative DAT). According to NICE CG98, what is the threshold for starting phototherapy based on this TSB level at this age?