What are my rights as a customer?

My partners aunt bought our daughter a dress from M&S for her first birthday. She wore it for the whole of 10 minutes before she got banana all over it and I had to wash it. I washed the dress to the exact directions on the label (inside out and not above 40 degrees) the stitching tore just under one of the ruffles, the receipt was not kept. What are my rights when returning this item? Will they replace it, or give me store credit? They are a very big chain and pride themselves on excellence, so I assume that they do not want any kind of bad service.