Complex sentence: In the afternoon of 19 July the 53rd Brigade advanced from the base of the salient to attempt to reach Thackeray's headquarters and reached the base of the salient but were unable to support to the forward elements of the South African brigade.
Split into simpler sentences: In the afternoon of 19 July the 53rd Brigade was pressed forward, through the base of the salient to attempt to reach Thackeray's headquarters. They succeeded in reinforcing the base of the salient but were unable to provide any meaningful support to the forward elements of the South African brigade.