Digital Essence's post in I've disabled stock management but still showing "out of stock" was marked as the answer
El Patron very kindly worked with me last night (it was the morning for him I think) and resolved this.
It was the theme I was using which hasn't been qualified for 1.6, As soon as I put it into the default theme the stock messages didn't show.
I should have thought of this earlier as it's the first thing we do when testing the DITA XML system I work on in my main job but my excuse is I'm choked with the flu and my brain is working properly.
Anyway, big thanks to El Patron who gave up his time and expertise to help a complete stranger.