One final possible clue; my wife, Ernest Parfitt's granddaughter, says that she vaguely remembers one of her grandads saying something about 'being in Russia' when in the army. Her other granddad was in the Manchesters early on and then transferred to the Monmoutnshire Regt in Jan 1916, and there is nothing on his army record about Russia.
She was a very young girl at the time and cannot be certain that the memory is accurate or not, but it's something else to consider.