Quote your consumer rights to them (politely)
To quote from the Money Saving Expert (UK) website
The item must be of Satisfactory quality, As Described, Fit for purpose, And last a Reasonable length of Time.
Within six months: The shop must prove the item wasn't faulty when the transaction took place. After this period, you must prove it was.
If the item breaks the SAD FART rules above, your agreement's with the shop you bought it from, NOT the manufacturer. So the retailer MUST deal with it - don't let it palm you off.