Builders Warehouse expects me to take back a battery which does not work!!!

In October I advised you of my experience at Builders Warehouse, and I was delighted, with the immediate response. It was in connection with the inverter which I had purchased, things have, however, gone downhill. After the battery was returned to them, I was notified that they had erroneously not sent the item to be checked, but would do so immediately. Yesterday I was again contacted by Cliff from Builders warehouse and he stated he would return the battery, but I would have to buy a new one, his only explanation being the distributor stated it was not working. Please assist.