With the exception of 'expendable' items, such as printer cartridges, you have the right to demand a refund or replacement of any item which fails due to an 'inherent fault' for a period of 6 years. (An 'inherent fault' is something which was wrong at the time of purchase, such as the use of poor quality components or poor workmanship).
After 6 months from the date of purchase it's up to the purchaser to show that the most likely cause of a failure is the presence of an inherent fault. However, for the first 6 months from the date of purchase there's an automatic assumption, in law, that any failure must be due to an inherent fault (unless, of course, there's clear evidence to the contrary. e.g. marks showing that the item has been chewed by a dog!).
Your statutory rights are not voided by the absence of a receipt. (Although, of course, proof of purchase can make matters simpler).
I've not tried taking stuff back to PC World but I took a faulty laminator back to Sainsbury's, several months after purchase and with no receipt, and got a replacement with no fuss whatsoever.
Give it a try!
Chris