Jamie,
As you will know, on the afternoon of the 1st July 1916 the 20th Manchesters went on the attack on the Bois Francais (Fricourt) front. Sassoon's battalion, the 1st Royal Welch Fusiliers was in support - Sassoon called them the "Flintshire Fusiliers". As regards Sassoon, he was left out of the battle, but was able to watch the fighting (and later comment upon it) from a high piece of ground overlooking the battlefield.
His eye-witness account of the Manchesters' attack is very interesting and mentions the sad death of Colonel Lewis the Manchesters' CO. PhilipG.