Whenever you buy something, you automatically enter into a kind of contract with the trader, even if the deal has just been done verbally. The Sale of Goods Act 1979 legislates a strict set of rules which retailers and other merchants must adhere to. Under the act all goods must be 'as described', 'fit for purpose', and of 'satisfactory quality'.
If you buy something, it must be of equal quality to any sample you were previously shown, as well as matching it's written description. Any goods you buy must be fit for their everyday purpose, as well as any specific purpose you agreed with the merchant. For example you may have been sold something on the advice that it's compatible with your computer, only to find out that it isn't when you actually come to install it.
Who's To Blame?
If you're in receipt of something which you believe violates any of the above criteria, then the trader is in breach of the Sale of Goods Act, and you can make a claim. You should note that your rights under the act are with the merchant, and not the actual manufacturer, and therefore you need to try and redress the issue with the person or store which you bought it from.
Return, Replace or Repair?
With the Sale of Goods Act, there is no time-frame stipulation for when an item has to be returned by. Instead you have to rely on your own intuition as to what might be deemed reasonable. Obviously this will vary depending on the product and the fault; you may receive an item that is damaged or doesn't work, or it may be an electrical item you've bought which breaks down after a couple of months.
If you want to reject an item and send it back for a refund, three to four weeks is generally accepted as a reasonable amount of time for most items. This is further backed up by the introduction of the Consumer Rights Act in October 2015, which stipulates a rejection period of 30 days.
If you have passed the rejection date, you have the right to get the item repaired or replaced. The retailer has the final say on what course of action will be taken; needless to say, they'll do whatever is cheapest.
If the merchant can't repair or replace the item either within a reasonable time, or without significant inconvenience, you're entitled to claim a partial refund on the purchase price, or a full refund, minus an agreed amount to cover the usage you had from the goods before failure.
If the retailer outright refuses to repair or replace the goods, you're entitled to have a third party fix the item at the cost of the merchant, but you'll have to claim the money back.
After Six Months
Within the first 6 months of the purchase, it's up to the merchant to prove that the goods weren't faulty at the time of sale. However, after 6 months the onus switches to you. If you have a dispute which can't be settled in a civil manner, you may need to take the case to court, and under the Sale of Goods act, you have up to six years to do this (five years if you live in Scotland).
If you take the case to court you will most certainly need to provide enough evidence to convince a Judge, but proving that the fault is due to poor quality and not misuse isn't as difficult as it first sounds. Depending on the nature and value of the goods it may be necessary to enlist the help of an expert, such as an electrician or mechanic. Failing that you may need to just collate enough evidence from other customers, which indicates that there is a problem with that particular product line. Searching Google and looking through the various forums it throws up can reap huge rewards when looking for such evidence.