My job up until I retired involved going into peoples houses, and you would be gob smacked if I told you the amount of folk that have had the same problems as you with a sofa.
They will always say what they have said to you, in other words try first of all to fob you off, just to test if you will take it any further.
They know the whole process of forcing them to do something about it is stressful and time consuming, making you prove that's its (not) fair wear an tear. Trading standards is your best bet, that's if they have the staff.?