I have a '54 Carens which has exactly the same problem, which leads me to think there may be a question mark over its build quality. However I haven't found any way to replace the cord save for dismantling the seat and re-attaching the cord. I findit easier to carry a long, flat-head screw driver which I insert in to the gap near the seat hinge and use to flick the mechanism so the seat breaks. Its fiddly at first, but after a few goes you finally get the knack.
Sorry its not the ideal solution, but it works and in the absence of any kind of owners/workshop manualits the best solution I can offer.