A retailer is obliged to repair or replace an item which becomes defective due to an 'inherent fault'. (i.e. something which was actually wrong with the item at the point at which it was sold, such as the use of poor quality components or poor workmanship).
Up until SIX MONTHS from the date of purchase there's an automatic assumption that any problem which develops must have come about due to such an inherent fault (unless, of course, there's clear evidence to the contrary, such as the tyre tracks of the vehicle which ran over it!).
Therefore, within that 6 month period the purchaser isn't required to prove that there was something wrong with the item when he bought it; the law simply assumes it, enabling him to demand a repair or replacement.
After the six month period is up the retailer CONTINUES to remain responsible for repairing or replacing the item if a problem occurs due to the presence of an 'inherent fault' but it's then up to the purchaser to show (possibly through obtaining an independent report) that there was indeed something wrong with the item when he bought it.
So, at that stage (given the likely cost of obtaining an independent report) a customer might prefer to seek redress from the manufacturer under the terms of any guarantee offered by that manufacturer.
A customer might also want to call upon such a guarantee if the retailer has gone bust (even if that occurred very shortly after the purchase was made).
It should be noted though that, while the law clearly defines a customer's statutory rights with the retailer, any guarantee offered by the manufacturer can be as limited or as generous as that manufacturer chooses. The manufacturer doesn't have to provide any guarantee at all. If he chooses to do so he's fully entitled to state "A refund, repair or replacement of any defective product will only be considered if the purchaser delivers the item in person to our factory in China, at 3am on the second Sunday after a full moon, while stark naked, walking on his hands and singing the Croatian national anthem backwards".