You might have left it too late by a few weeks (but it's still worth a bit of hassling).
EU law requires that, where a problem with a purchase occurs due to an 'inherent fault' within 6 years of purchase, the retailer must provide a remedy, either through a repair or a replacement. (An 'inherent fault' is something that was wrong with the products at the time of purchase, such as the use of poor quality materials or poor workmanship).
Within the first 6 months of purchase there's an
automatic assumption, in law, that any problem that develops must have come about due to an inherent fault. (Unless, of course, there's clear evidence to the contrary). So, up until a few weeks ago you could have simply said to the retailer "It's faulty. Repair it or give me another sofa". (The retailer could have chosen to give you a refund but you would have had no right to insist upon it).
After the 6 months is up (and for the remainder of the 6 year period) you still have the right to a repair or replacement if the problem has arisen due to an inherent fault but you have to prove that such a fault is the most likely cause of the problem.
Complain to the retailer, stating that you're confident that the problem has only come about because the sofa wasn't of a satisfactory standard when they sold it to you and invite them to suggest any other reason why the problem might have occurred. (If they can't, it's virtually an admission that it was a poor-quality product that they sold to you, which can add to your bargaining power).
If you don't get satisfaction, contact your local Trading Standard department. Use the postcode facility here to get their contact details:
http://www.tradingstandards.gov.uk/advice/index.cfm