Complex sentence: After the removal of the lead from the roof in 1567, the cathedral fell steadily into decay which was only arrested in the 19th century when it was in a substantially ruinous condition.
Split into simpler sentences: After the removal of the lead from the roof in 1567, the cathedral fell steadily into decay. Its deterioration was only arrested in the 19th century, by which time the building was in a substantially ruinous condition.