Complex sentence: Burston remained in hospital until September 1916, when he was posted to the 11th Field Ambulance, part of the 3rd Division, then training at Larkhill in England.
Split into simpler sentences: Burston remained in hospital until September 1916, when he was posted to the 11th Field Ambulance. This was part of the 3rd Division which was training at Larkhill in England.