hi gritty, just had a little search this morning and found,
CLARK
Initials: B W
Nationality: United Kingdom
Rank: Private
Regiment/Service: Essex Regiment
Unit Text: 11th Bn.
Date of Death: 28/08/1916
Service No: 23581
Additional information: Husband of H. Clark, of 63, Linford Rd., Walthamstow, London.
Casualty Type: Commonwealth War Dead
Grave/Memorial Reference: II. D. 13.
Cemetery: COUIN BRITISH CEMETERY
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in grave next to him his,
CASEWELL
Initials: H
Nationality: United Kingdom
Rank: Private
Regiment/Service: Manchester Regiment
Unit Text: 7th Bn.
Secondary Regiment: The King's (Liverpool Regiment)
Secondary Unit Text: attd. 1st Bn.
Date of Death: 28/08/1916
Service No: 4780
Additional information: Son of Mr. J. Casewell, of 81, Booth St. West, Chorlton-on-Medlock, Manchester.
Casualty Type: Commonwealth War Dead
Grave/Memorial Reference: II. D. 12.
Cemetery: COUIN BRITISH CEMETERY
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and the other side of him ,
WRIGLEY
Initials: H
Nationality: United Kingdom
Rank: Private
Regiment/Service: Manchester Regiment
Unit Text: 10th Bn.
Secondary Regiment: The King's (Liverpool Regiment)
Secondary Unit Text: attd. 1st Bn.
Date of Death: 28/08/1916
Service No: 2786
Casualty Type: Commonwealth War Dead
Grave/Memorial Reference: II. D. 14.
Cemetery: COUIN BRITISH CEMETERY
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FENN, JOSEPH
Initials: J
Nationality: United Kingdom
Rank: Private
Regiment/Service: Manchester Regiment
Unit Text: (2358). "C" Coy. 2nd/10th Bn.
Secondary Regiment: The King's (Liverpool Regiment)
Secondary Unit Text: attd. 1st Bn.
Age: 25
Date of Death: 28/08/1916
Service No: 375606
Additional information: Son of Henry and Jane Arm Fenn, of Millgate, Hollins Rd., Oldham; husband of Lily Ainsworth (formerly Fenn), of 3, Top Vine St., Hollinwood, Chadderton, Lancs.
Casualty Type: Commonwealth War Dead
Grave/Memorial Reference: Pier and Face 13 A and 14 C.
Memorial: THIEPVAL MEMORIAL
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so the grave is not unknown ,
your great uncle died along side these lads and could be in a unknown grave in this same cemetery ,
hope this helps. tonyrod