Thanks everyone for your help, this has been a interesting mystery for me..
I still can't comfirm William's 'so called' Death during WW1, but I have managed to find out a bit more about him and his wife Margaret.
I've learned by contacting the Durham Record's office that William did serve in either the 14th or 15th Battalion (21st Division) and that he first saw action in France & Flander on 11 Sept 1915, (which is also the date on his medal card), on the 25 Sept 1915 he was in the Battle of Loos, and we believe this is were the information of him been gased comes from.
As for his Marriage to Margaret McGowan, while looking through the National Archive Records I came across a record for a filed Divorce between William and Margaret with a Robert Todd Morton as a Co-respondent.
Looking at what you've all said above and doing a little digging on Ancestry and the BMD I've found this information out...
Margaret Ann McGowan married Robert Todd Morton in Apr Qrt of 1918 (South Shields, Durham)
William Files for divorce in 1918 (date unknown as waiting for documents to come in post)
Margaret then re-marries Robert Morton in Sept qrt 1920 (Birkenhead, Cheshire)
Margert has three children to William Graham...
Henrietta Jane (1910-1983)
Margaret A (1912-1989)
Wilhelmina Henrietta (1914-2010)
And seven to Robert Morton, two of which are while she is still married to William...
Joseph M (1916-1987) Registered under both Graham & Morton.
Gladys M (1918- ) Registered as Graham
Doris (1921- )
Edna M (1923- )
Hilda (1925 - 2012)
Lily (1926- )
Jean (1929-1935)
Margaret must of heard about Williams death in the late months of 1915, and then took comfort with Robert Morton in the Jan of 1916 as Joseph was born 12 Oct 1916.
I can understand why she did this with three young children to feed, but I really do feel sorry for William she didn't grieve him long
.
I'm hoping the divorce papers will shed a bit of light on what happen, and I'll post the finding.
Thanks again to everyone who helped
Clare x
William Edward Graham
Margaret Anne Mcgowan
Robert Todd Morton