Car not working properly after repair

I took my car to a garage (Midas in Ghent) because it was overheating. They replaced the water pump and various other things and was quite expensive. I asked them before picking it up if they checked for mixing of oil and water or other problems and they said they did and the car was working properly. When they gave me the invoice they said to keep it as proof of guarantee. After driving it for 100 km it overheated again, and I am taking it back this afternoon and will press them to repair it under warranty. But from previous experience I know that garages try to argue that it is something else and that their guarantee is only for the parts they replaced and for proper placement. How best to handle this and if they refuse to repair under warranty, what recourse do I have?