I think they can tell which ones are faulty by the model numbers so you shouldn't need a receipt if you have one of the faulty batch. I'd give Argos a ring first and check though rather than lugging the gate back to the store.
Oh yes, and you don't always need a receipt as proof of purchase. If you paid by card then bank statement could suffice. Also if the item broke because it was faulty you could still have gone back to Argos (as the supplier they are directly liable) or failing that contacted the manufacturer. However, in this instance, the item has obviously come from a faulty batch so you shouldn't have any probs getting a refund.
Diva, i had this email too the stairgate i have is one thats effected. I phoned argos and they told me that as its being recalled you dont need a reciept. If you havent got it then they will issue you with a voucher to purchase a new one.