Complex sentence: Sieff returned to Britain in 1951 to take over the Marks & Spencer food department, which he expanded from 14% of total sales in 1956 to over 25% in 1968.
Split into simpler sentences: Sieff returned to Britain in 1951 to take over the Marks & Spencer food department. Under Sieff's management the Marks & Spencer food department expanded from 14% of total sales in 1956 to over 25% in 1968.