Hello Robin,
I dug out the SWB list myself yesterday and also noticed that Ernest and every other man on the page had been discharged on that date. On the page before, the SWB recipients had been discharged the day before.
That is very peculiar, unless perhaps these men had been wounded just before the Armistice and were in hospital at the time. They would no longer have needed an SWB to demonstrate to the wider public that they had served King and country, and it may have lightened the burocracy to discharge them all on the same date. However, this is all pure speculation on my part.
I have no idea how quickly men were discharged following a serious injury.
I would urge you to join the Great War Forum
http://1914-1918.invisionzone.com/forums/index.php and pose the question to the real experts. I'd be interested to hear what you discover.
To address your original question, have you thought of hiring a private researcher? It would surely cost less than £500. If your ggf was injuried in the weeks running up the Armistice, the researcher would not have to spend too much time looking through the war diary.
Justin