Complex sentence: The Ottoman garrison would be eventually encircled by the two infantry and two mounted divisions as they (and their supporting artillery) moved forward after the 60th (London) Division captured the redoubt on Hill 1070 to bombard the main Ottoman trench line.
Split into simpler sentences: The Ottoman garrison was eventually encircled by the two infantry and two mounted divisions, as they (and their supporting artillery) launched their attacks. The 60th (London) Division's preliminary attack and capture of the redoubt on Hill 1070, led to the bombardment of the main Ottoman trench line.