Personaly I would be straight in the car at the shop ranting and raving like a mad man till I got satisfaction or they called the cops to get rid of me after of course I'd exaushed all the proper routes of complaints. I'm having to return a Graphics Card very soon that keeps crashing my computer all the time, Im hoping for a quick refund, no fuss and I'll buy a differnt card from them with the money but if get messed about at all I'll be straight on the phone to traiding standards, in my car at the shop with some of my mates and be kicking off like some sort of mental patiant all day if I have to.
They have a legal right to replace your item with the same or better and if they dont make sure you complain to trading standards, citizen advice bureau and go to the shop and dont just make a small fuss, kick off like your on somthing.
Im not saying do it right away, as a last resort of course, after you have been through all the normal channels. Bending over and taking it gets you no where these days, if they mess me about with my return belive me there going to really know about it like nothing they know.