Charlie and Sphinx,
Thank you very much for these helpful comments.
I have looked at the two medal rolls before and nothing jumped out at me as being absolutely clear, but on Charlie's recommendation I will go back and look more carefully. I would be interested to know the kind of routes that led to a Londoner joining the Manchester Regiment - I can imagine that it might be chance, but has anyone any comments?
In response to Sphinx's helpful research in the 1901 Census, I ought perhaps to have stated more clearly that I have a good deal of other non-military information about AGS from the C19th Censuses and elsewhere and this includes identifying him living in London in the 1901 census with some certainty. Of course, this date has implications for his possible involvement in the Boer War - though there is an A. Stagg who served with the 1st (Royal) Dragoons in South Africa, which is why I asked about men moving between regiments at this time.
The key fact I hang on to is that I find it hard to believe that the Last Post Organisation in Canada, a semi-Government agency largely staffed by ex-military personnel, would be casual in authorising any inscription on a gravestone. And this seems especially relevant to the figure 4 following the 2nd Batt Manchester Regiment 4. This must have meant something to them - or they must have been working from sort sort of official documentation. I have written to them but they have no remaining records for this period.
With thanks and best wishes to my helpful advisors.