It seems that there is no disagreement about the sofa being faulty and the vendor being liable to remedy things for you. The options open to the vendor are to repair, replace, or refund your money. The courts have traditionally taken the view that the vendor can choose the remedy and (generally speaking) is allowed one shot at each option.
I presume that everybody agrees that a repair is not reasonably manageable, so the vendor opted to replace. So far, so good.
The attempt to replace has not worked so far. The question is now one of reasonableness: can you reject the proposed replacement because it is a couple of weeks later than promised? Are they really letting you down, or are they the victims of circumstance?
In your position, I would hold back on escalating. I would give them written notice that unless they deliver the replacement within five working days, you will reject it and proceed against them for a refund.