Complex sentence: Demolition work was complete within the year, except for one porticus and the great altar and the following year, more relics "of St. Swithun and of many other saints" were found under that altar and translated to the new church.
Split into simpler sentences: Demolition work was complete within the year, except for one porticus and the great altar. The following year more relics "of St. Swithun and of many other saints" were found under that altar and translated to the new church.