Builders Express are not honouring the product Guarantee and are unprofessional in the handling of the situation

I bought 2 Hosepipes with a 4 year guarantee on the product from Builders Express Cresta. One I returned recently as it became faulty. In terms of the Guarantee i requested that the hosepipe be replaced. The person at the front desk refused to replace the hosepipe and called the store manager Macdonald. He said they have 21 days to sort this out. It is now 27 days and many calls later I have been told that they are trying to speak to their buyer???? Clearly on the packaging it says that it has a 4 year guarantee. There issue is the product has increased in value and is now more expensive. They also did not take my details down as per standard procedures and made a decision themselves to give me a credit on my customer card which is also wrong. They refused to give me the products packaging back which was my only proof of my guarantee. Fortunately I took a photograph of the detail. This is not the way to honour Guarantees and treat customers. They are also refusing to give me there email addresses