When I did the refill on my Clio, I used some Halfords brand stuff, as long as the ratio with water is correct youll have no trouble. One thing tho, depending on how long you have had the existing coolant in your engine, it is best to do the job properly and flush the system first as I have ben told that mixing coolants isnt the best idea!! Especially if your topping up coolant that has been in the system for a while - or of a different type (make)
As for filling... you fill up to the "MAX" level while the engine is cold with the mix, and then run the engine til warm!! This allows the coolant to circulate through the engine. Let the engine cool, and then top if required with water.