I bought a camcorder from Argos but dont have receipt want to return it for vouchers is this ok please read?

I bought this as a present a few weeks ago with cash and dont know what i did with the receipt. The camcorder was originally �200 but i got it on offer for �170. The camcorder looks ok but there is another camcorder i have seen which looks far better it a HD Camcorder worth �400 and its currently on offer for �200 (half price). Iwant to get this instead which i know you are not allowed a refund as it is excluded from the 30 day money back guarantee unless it is faulty. So im going to say its faulty and because i dont have a receipt i hope hey will give me vouchers. I know argos are generally ok but i dont want them to say that they will send it away to get it fixed. If they offer to replace it could i tell them i want to pay extra money for another model as im afraid the same thing might happen again?? would this work cos i already have tjhe other camcorder reserved but at another store. What excuse do i use for it not working? Argos product code sticker is still on the box??