Complex sentence: The tower and parts of the church date from the early 16th century but the church was restored and largely remodelled in 1869--1870 by John Douglas during which time the remains of an earlier church which had been destroyed by fire were discovered.
Split into simpler sentences: The tower and parts of the present church date from the early 16th century. During this time the remains of an earlier church which had been destroyed by fire were discovered.