In providing redress to a consumer, a retailer is entitled to satisfy himself that the product was purchased at his store and on the date claimed. A sales receipt is a good way of providing such proof (as is a well detailed credit card statement). Although sales receipts are not a legal requirement, consumers are advised to request them where they might later be needed and to keep them safe.
Consumers do not have to accept credit notes if goods do not conform to the contract. However, they may be offered where the consumer has no legal right to any redress but the retailer wishes to be helpful e.g. when the consumer has a change of mind.