Hi Justin
Ok, in answer to your questions, there are lots of johns, which is good and bad for this kind of thing. The John that is commemorated is probably Leonard's brother, my great uncle. I would be interested to know where the memorial can be found, as I have never seen it. I thought he was a bit old too, he would have been in his 40s and yes he was born around 1870, as his wife was born in 1867, but she did die in 1915 and he might have volunteered, I don't know. Leonard did serve, he was awarded the victory medal and another I can't remember the name of. I didn't save the source for their marriage, so am going to have to find it again, but they seemed to be quite recently married in 1891, as the census say that their eldest son, John is just over 7 mths old at the time and they are living in Droylsden at that time, which makes sense, so I think they got married around 1889/90, they are then seen in 8 garner st by 1901
Garner st no longer exists after the urban renew, it is now the car park of the health centre sadly, but they did name a close after the Flints, so there is a Flint Close, the whole area is a dive now, from what I have seen a few years ago, I no longer live in the north, so have not been there for a long time.
If you go on wikipedia photos under clayton, there is one photo of the corner of 8 Garner st and Flint st I think. If you have probs finding it, tell me and I will see if I can see it again. I would not be surprised if it did not appear in a publication to do with old clayton, if one exists.