Today my replacement seat from Ocean Cycles arrived.
The original was returned to them because it had visible cracks/splits across both rear ribs.
I carefully opened the packaging only to find that this seat has exactly the same damage as its predecessor to one of the ribs.
Indeed the area around the fracture is so thin it is possible to depress the rib at this point with light to moderate finger pressure so I am not surprised that it has cracked/fractured.

I am so surprised that simple post-production checks for robustness would not have called out the weakness prior to packing and shipment.

I am somewhat disappointed. If it is this weak, what would happen when a recumbent rider is really putting some pressure on the seat during riding?

As I have already paid for the seat plus a further £12 for the return of the last faulty one I am inclined not to be paying any more and may ask for my money back if I do not get a satisfactory response.