A 58-year-old man on ICU develops unexplained hypernatraemia (Na+ 158 mmol/L) despite receiving 0.9% saline. His urine output is 4.5 L/day. Urine osmolality is 150 mOsm/kg (dilute). Serum osmolality is 325 mOsm/kg. He is post-neurosurgical and has no access to free water. What is the most likely diagnosis?