Up until the point at which you're deemed to have legally 'accepted' a purchase, you're entitled to a full refund of your money if that purchase is defective. (Thereafter the retailer is only obliged to repair or replace an item if a defect, due to an 'inherent fault', occurs within 6 years).
Since you spotted the problem straight away, and have never agreed that you've received a satisfactory product, you've not accepted' the defective sofa (in the legal sense of that word). Accordingly, you're entitled to a full refund.
Chris