Thanks for these further suggestions/thoughts - I appreciate them.
Yes, Nick29, you are so right about the poor quality of my grandfather's clothing: I ?reckon this photo was taken just after WW1 (which he survived as a soldier in the trenches). I'm fairly certain that his wife and children were living in a workhouse while he was fighting in WW1, and continued to do so after he returned. Eventually, he earned enough money for them all to be reunited in a rented house in Leatherhead.
For clarification: the first official bit of info I have on my grandfather is in 1900; it's London, and he's enlisting with the 1st Battalion, Leinster Regiment. He says on his enlistment papers that his name is James Aloysius Smith, that his next of kin are his older brothers David and John, address unknown. On later official documents (his marriage certificates), he gives his father's name as Henry Arthur Smith, farmer, deceased; and Henry Arthur Shaw, farmer, deceased.
He had a Canadian passport in the '50s; but having checked out the Canadian Passport site, it would appear that they don't keep records of 'old' passport applications, and also that there were no stringent checks for people applying for passports prior to 1950 (ie my grandfather could have applied for a passport without providing verification - but this still begs the question: if he wasn't actually a Canadian citizen, then why apply for a Canadian passport? ie what's in it for him to be a Canadian, if he actually wasn't one?)