Five year guarantee Padlock packed up after two years

We all know that Build It is is overpriced with ALL their goods. But I still buy there as it is near to me. The padlock I bought had a five year guarantee and it broke. When taking it back they want the slip, which I don't have anymore. They want me to pay for a new lock again. Looking at prices Build It merchandise is at least 50% more that Kit Kat or any other store. What a mess.