Oh gosh, yes, FarNorth, Streets of London! That was one of my mother's favourites, and it's always brought a lump to my throat too.
I just had to find it on YouTube and listen - I found this recording. It includes a short interview with Ralph McTell, in which he explains that he wrote the song originally about Paris, but changed it when he found there was already a song about the poor people of Paris. And of course, as he says, the words could apply to any major city in the world. But he was in Paris at the time, and it was the poor of that city which inspired the song.