With the new information found by SmallTownGirl, his service has fewer mysteries and I suggest a scenario as follows.
Trained at Minster (Kent) with 5th Battalion Rifle Brigade (RB) and is sent to France in December 1916 to join 3rd Battalion RB. (5 att 3 R Bde A means his record showed him still on the books of 5 Battalion but now attached to 3rd Battalion. A is the company he was in).
3rd Battalion RB diary shows a draft of 9 men joining on 18th December which would be about right allowing for a short period on arrival to pass through the base depot.
In December 1916 to March 1917 3rd Battalion RB were in and out of the line on the fairly normal five or so days in the trenches and five out. While out they still were employed labouring in various parts of the rear lines. The areas they were deployed in was around Loos with their "rest" billets a little to the west.
On 14 March 1917 we have the record of him being in hospital at Le Touquet essentially with slight exhaustion. We don't know when he left and returned to 3rd RB or whether he joined 2/10th London regiment straight away.
In any event, his last unit overseas was 2/10th who were deployed a little further south in the area south of Arras. They spent the last week of March and almost all of April out of the line training or providing working parties. After the first week of May in the line they spent most of the remainder of May refitting and training. June was similar, few days in the line then a spell out of it. It was sometime in early June I suggest he was wounded and evacuated back to England.
While he was serving with the two battalions in France, no major actions took place in their sectors and casualties were in the ones and twos as both sides sat in the stalemate of the trenches.
As he does not have a Silver War Badge, he must have recovered enough to carry on serving in UK until discharge in 1918 or 1919. His medal entitlement was to the British War Medal and the Victory Medal.
There are war diaries from which the above is culled. They do not mention soldiers' names and are written for military readers. If you think you would like to read them, do let me know and I'll point you towards them.
Hope that gives a better idea of Grandad's service.
MaxD