Complex sentence: In Leeds, Marshall, an amateur musician, had sung in the choir of the parish church when, under S. S. Wesley, it was being brought close to cathedral standards, and served on the committee of the first music festival in 1858.
Split into simpler sentences: In Leeds, Marshall, an amateur musician, had sung in the choir of the Leeds Parish Church when, under the direction of S. S. Wesley, it was considered of a high standard. Marshall also served on the committee of the first music festival in 1858.