Complex sentence: Although they never truly broke into the mainstream, their most successful song was'XOYO' which came 41st in John Peel's Festive Fifty for 1982, which was the top 50 songs of the year as voted by the listeners.
Split into simpler sentences: Although they never truly broke into the mainstream, their most successful song was XOYO. In John Peel's' Festive 50' for 1982, XOYO came 41 in the top 50 songs of the year as voted by the listeners.