Jane,
Some further research.
Your great grandfather was killed, as Mack & Charlie have indicated, in an attack on Quadrangle Support Trench. He was taking a part in assisting with the attempt by the 38th (Welsh)Division to capture Mametz Wood; a terrible battle stretching over the period 5th to 12th July 1916. This battle has attracted public and media attention, because both Robert Graves and Siegfried Sassoon later visited the wood after its capture. In the case of Sassoon, he appears to have begun his famous walk to the outskirts of the wood at the junction of Quadrangle Trench and Quadrangle Alley, to which was linked Quadrangle Support.
Briefly, my source indicates that two attempts were to be made. The first was very early on the morning of the 7th July by the 10th Lancs. Fusiliers and the 9th Northumberland Fusiliers, but they were forced to withdraw, not least, because the German troops were themselves about to attack at the same time. The remaining two battalions of the Brigade - 12th Manchesters and 9th Duke of Wellington's (West Riding Regt.) - were ordered forward at 8 am that day. If the night attack by the two Fusilier battalions was not to succeed, it follows that this daylight attack over open ground would fail too, and it did so, with very heavy casualties to both battalions.
I realise that without a sketch map, it is difficult to picture the line of attack made by the 12th Manchesters. However, in one of the excellent photographs provided by Timberman (Reply 7), there is one of a path in the cemetery leading to the Memorial. Behind that is Mametz Wood, and I reckon from memory, that the direction of the Manchesters' attack would have been -taking the Memorial as being at 12 o'clock on a clock face - at the position of 10 o'clock.
To conclude. The authorities could not understand why the capture of Mametz Wood took so long to succeed. They were to realise the difficulties a few days later when High Wood was attacked, a battle which took from 14th July to the 15th September 1916 for it to be captured. Perhaps the Welsh and English battalions in the Mametz Wood battle were not so bad after all? PhilipG.