Complex sentence: The 1/7th Northumberland to the west of the 1/5th Northumberland may have captured Hook Sap but did not secure their part of Gird Trench; they were fired on from Butte Trench and disappeared.
Split into simpler sentences: The 1/7th Northumberland to the west of the 1/5th may have captured Hook Sap but did not secure their part of Gird Trench. They were fired on from Butte Trench and disappeared.