That article was very interesting to me, because I had no idea that the Inner City Mission had five members who were/became significant artists in their own right. The best-known name here at the time would have been Terry Black, who had had major hits here in 1964-65 with Unless You Care and Only Sixteen. But he then married Laurel Ward in 1970 and they started recording together, eventually having a #14 hit here in 1975 with Back Up (Against Your Persuasion). In 1977, Wayne St. John reached #11 (with the THP Orchestra) with Fighting On the Side Of Love. Colleen Peterson became a major Canadian country star in the 1980s, and Brenda Russell eventually moved back to the US and had a #6 Billboard hit in 1988 with Piano In The Dark. So the only member who wasn’t known to me was Toni Lark. 
And I remember the Inner City Mission, but for the #23 hit they had here in 1971 with a cover of Wild World. It likely got played here instead of Cat Stevens’ recording because 1971 was the year that radio stations here had to start playing 30% Canadian content recordings. However, I didn’t know that they had previously charted here with Get Back John until I heard it on an oldies program decades later. 
But the article says that the song was pleading for Lennon to return to give a concert here, and then says that he did return on Sept. 13 1969 for the Rock and Roll Revival show. However, since the song didn’t come out until April 1970, I have to wonder about this. I had actually thought when I heard it that it had to do with the breakup of the Beatles. But according to Wiki, the breakup didn’t become public until April 10, and as the article says the song entered the CHUM Chart on April 11. So I just tried listening to the song, but didn’t find the lyrics to be all that clear. However, I wonder if they just related to the fact that Lennon had visited Hawkins’ farm (which is definitely referenced), and was just about the possibility of him coming to visit again. The fact that the song dropped off the chart after 3 weeks is actually the same as what happened with Wild World, and I think this could be easily enough explained by the fact that it was by an obscure local group – and that despite all the talented singers involved, it just wasn’t very good. Actually, I felt that it sounded like too many singers were trying to do their own thing, rather than create a more cohesive group sound.