hiya peter.
its possible that your grandfather had been wounded on a previous occasion,or invalided home with sickness,while serving with the essex regt,on his recovery,he was transferred to the 1/8th liverpool regt.
he wasnt killed in action,he died of wounds at one of the casualty clearing stations at proven, near mendingham.
these were 4 of the casualty clearing stations that operated in that area, at the time of his death.
1/1st wessex CCS
46th CCS
12th CCS
64th CCS
he landed in france on 15-6-15,the only essex battalion serving in france around that date,was the 2nd battalion.
mack