Bednobs' 'repair or replacement' answer applies to goods which develop a fault (within a specified time period) AFTER you've legally "accepted" them.
The law allows you a reasonable amount of time to check that anything you purchase is of acceptable quality. The actual length of that period isn't laid down by statute. If you were to buy, say, a new hi-fi system you might need a few days to check that all of its various functions were properly working. A court has ruled that a woman who purchased a car in the winter months, and who didn't find out until the start of summer that the air conditioning was not working, had never legally "accepted" the vehicle.
Since you identified the fault quickly (and promptly notified the supplier) I have no doubt that, as far as the law is concerned, you never "accepted" the purchase. (Signing for it, upon delivery, is not the same as "accepting" it). In which case you retain the right to demand a FULL REFUND of your money. Of course the supplier may choose to offer you a repair or replacement instead but you have the right to decide whether such an offer is acceptable to you. (You might reject the offer of a repair but accept a replacement. Or you can reject both offers and insist upon your money back).
Chris