A 60-year-old woman is found to have a serum sodium level of 118 mmol/L on routine blood tests. She feels generally well with mild fatigue. Her medications include bendroflumethiazide 2.5mg od for hypertension. Clinical examination reveals euvolaemia. Urine sodium is 45 mmol/L and urine osmolality is 350 mOsmol/kg. TSH and cortisol levels are normal. What is the most likely cause of her hyponatraemia?