Don't mess with it, take it back. Most budget TVs are not very repairable these days.
Contrary to what some shops may tell you, by law, you don't need a receipt. Any proof that you bought the TV from them will do. In fact Asda will have proof in their shop because, by law, you will have had to fill in a form for TV Licensing when you bought it. Ask to see that form and ask to see the manager.
In addition how about a Credit Card Slip?
t, April 2008