Ive done my 1.2 and need to do my 172 soon. First of all when engine is cold remove filler cap, need to bleed it by disconnecting the lower hose from the radiator and the bleed screw on the plenun chamber. Bleed out all fluid. (at this point you could flush radiator out with hose pipe through the top rad pipe), put the lower rad hose back on but not the bleed screw. Then refil via the expansion bottle upto the max mark until it runs from the screw, then close the screw, then run the engine until its warm making sure it doesnt go below the min mark while topping up coolant. Now stop the engine and put the cap back on the expansion bottle.