You're probably referring to the Distance Selling Regulations, which gives you 7 days to return an item and receive a full refund no matter what the reason for rejection. You get that option when buying something online because you haven't had the chance to view the item first-hand. If the item is not available for viewing in-store, then I would suspect that you still have that right.
you are buying under the protection of the distance selling regulations as far as I'm concerned - the fact that the catalogue happens to be in a shop makes no difference. You have no opportunity to inspect the goods before paying, and should therefore have the right to return under the 7 day rule - no questions asked.
The company will give you 30 days to try the item out in your own home. There are very few stores that you can go and view furniture...currently only Mill Hill and Cardiff Bay have some selected items in store (I'm making the assumption that you are talking about a sofa or some such item).
If you aren't happy with the sofa within 30 days, then go into your local store to arrange collection and a refund, but bear in mind the refund won't be given to you till the item is back in Argos Direct's warehouse. The item has to fulfill the same conditions as everything else, ie, be in it's original, undamaged condition.
If it's white goods that you're talking about, then it's slightly different, as they're exempted from this 30 day guarantee, make sure to check the catalogue before you buy.