Figgs, it was the mouth that made me stop and look at the image more closely to begin with. I have been sending the two pictures to friends and family for comparison as well as posting on here, and there is still an element of doubt. (The doubters all have a problem with the ears, by the way).
I think it may help if I can find other pictures of my grandfather.
Having slept on it, I think they are possibly one and the same. What would be the chances of him having a doppelganger in the same battalion? A lot of first cousins served in the same regiment during WWI but he was the only one in that battalion at that time. So, that is quite a strong argument for it being the right person, but if it is, it would indeed be a 'fantastic find'.