How did you pay for them? If you paid by Credit/Debit card then you can take your statement in as proof of purchase.
If you paid by cash then you don't have much of a chance unfortunately - yes the retailer is obliged to reund if goods are faulty, but they also have the right to ascertain that the goods were purchased from them in the first place, as opposed to another shop.
If you can remember when you bought them you could ask the shop to look back through their audit rolls (or electronic audit roll) to track the transaction, but it depends whether their systems are sophisticated enough for this sort of search to be carried out.