By selling you an MP4 player that doesn't play MP4 files, it sounds like Argos have breached the Sale of Goods act, particularly section 14, which states:
(2) Where the seller sells goods in the course of a business, there is an implied term that the goods supplied under the contract are of satisfactory quality.
(2A) For the purposes of this Act, goods are of satisfactory quality if they meet the standard that a reasonable person would regard as satisfactory, taking account of any description of the goods, the price (if relevant) and all the other relevant circumstances.
(2B) For the purposes of this Act, the quality of goods includes their state and condition and the following (among others) are in appropriate cases aspects of the quality of goods—
(a) fitness for all the purposes for which goods of the kind in question are commonly supplied,
(b) appearance and finish,
(c) freedom from minor defects,
(d) safety, and
The goods are plainly not as described (section 2A), nor are they fit for the purpose for which they were suppled (section 2B(b)). If the goods are not of satisfactory quality, then your right as a consumer is to terminate the contract - to return the item to the seller and to ask for your money back.
The language used makes it clear that your contract is with the seller, so Argos are bound by the SGA to make sure their goods match the description, and similarly bound to offer you a refund if the goods do not. Argos cannot contract out of this, as the Unfair Contract Terms Act 1977 states. See the answer here for more detail on this.
As to what you should do if the shop staff have a problem with this: feetwet is right to say that we can't advise on that here. (Nor should this answer be taken as legal advice - it's merely one person's interpretation of the statute.) Your best bet, should you come across any problems, would be to seek legal advice, perhaps from somewhere like your local Citizens Advice Bureau.