Contact the retailer you bought it from. Tell them about the problem and ask for a replacement, or arrange to have it repaired.
Ultimately, the retailer can choose which of these to do - normally it will choose the cheapest option. But to begin with, you can ask for whichever  option you'd prefer.
If the product is within its guarantee period, check the terms of the guarantee. Find out if the manufacturer will offer a repair or replacement.

Top Tips

  • Contact the retailer - your contract is with the retailer and not the manufacturer
  • Cite your rights under the Sale of Goods Act
  • Don’t be fobbed off - you can always take the retailer to the small claims court

If the retailer or manufacturer will not help, write to the retailer (not the manufacturer) saying that, under the Sale of Goods Act, the item is not of satisfactory quality and you are exercising your right to have it repaired or replaced.
If it continues to fail to offer a remedy and you want to resolve the dispute through court action, start following the steps in the Practice Direction on Pre-action Conduct.
If you get no response from the retailer or if it has gone bust, and you paid for the item with a credit card, you can take your claim to your credit card company.
You have the same rights from your finance provider as you have against the retailer. .