David.
You lucky man. What a fantastic find in the old loft. Magic. (Definitely need a
GREEN WITH ENVY SMILEY THINGY)
Do you know which of the Battalions your Great Grandfather served with. If it was the 25th, and he was an Officer or NCO, I may have him in a photograph, I doubt if it was though, because I don't believe the 25th went to France.
However, I suspect that as he served in France, he was either 18th; 19th or 26th.
Now having re read your post, it may be that he was a Pioneer whilst recovering in the UK, which may mean that he was in the 25th.
Whatever and wherever he was, it's a great story.
I'd love to see a piccie of the badge, as I'm sure a lot of members would, with a provenance like yours it will help dispel a lot of myths and show good from bad.
Regards
Brian