Apparently there were two farming families in the valley who had an ancient feud. One family had a daughter and the other a son who, of course, fell in love. When the respective families found out about the love affair there was a huge scene and the pair were told they would never be allowed to marry.
So the fated lovers met secretly one last time. They climbed the fell where the wood is (though there wasn't a wood then) and he killed her and then killed himself.
Both families were so distraught at the tragedy they had caused that they planted the heart shaped wood at the place where the pair had committed suicide.