The T9’s cooling system has multiple high points and is prone to air locks. The workshop manual explicitly states that traditional "fill and run" methods will fail. It mandates a vacuum filler tool—and gives you the exact vacuum pressure to apply.
Detailed repair procedures for the various powerplants available in the T9 generation, including the 1.2L PureTech (Petrol) 1.6L BlueHDi (Diesel) Mechanical: Peugeot 308 T9 Workshop Manual