Sofa lists to one side under weight
A seat that drops 20 mm or more to one side when sat upon has a deformed or broken sinuous spring on that side, or jute webbing that has stretched past recovery beneath the seat deck. The leather above may be perfectly sound. The problem is entirely structural - inside the frame. A frame repair reupholstery strips the seat deck, identifies and replaces the failed spring, relays the webbing and refits everything. Check our leather glossary on sinuous vs 8-way hand-tied spring construction.