Thanks guys much appreciated.
The one chord arrangement is band in a box. I fed different versions into BIAB to see the chord structure tempo etc. But I wasn't happy with the sound as it was too dull. I kept knocking chords out & changing styles & increasing tempo until I felt it was more fun.
I have never had much joy with commercially available backing tracks as I feel most are too overpowering for the harmonica. I don't think IMO the harmonica needs full orchestral backing (at least not on my playing) but just some simple strumming accompaniment is enough.
Maybe, as suggested, I could rework the lyrics and truly make it a happy tune. We'll see?