Extract from KORLR reply - nominally unable to add to what is known, but helpfully noted:
"... Private Robert Barnes went overseas to Gallipoli (not Mesopotamia) on 13th June 1915, with the rest of the 6th Battalion. He is recorded as 'dead' rather than "died of wounds" which suggests to me that he died of disease probably contracted in Gallipoli. He would have been evacuated to Egypt where he later died. The 6th Battalion had no presence in Mesopotamia until 1916."
Spent another two and half hours at the library tonight and rechecked both papers for 4 months after date of death without luck. I think this is going to be one of those were we come across some information when we are not looking for it (I hope you know what I mean).