I ran the Admin > Settings > Dangerous > Delete all the tracks, and deleted manually the files from the filesystem too so everything would be fresh. I reimported all the tracks into the database… to be honest there weren’t that many so it wasn’t such a task. Once I went back into the calendar something odd had appeared, there were two ‘shows’ that apparently started broadcasting in March 2018 and ended today… For whatever reason these shows may not have had an end date set in the database (import failure or other database wierdness) and as such may have been interfering with subsequent scheduled content. Presumably these were ignored under 2.x but caused MADness under 3.x
In any event, when I tested it the scheduled tracks from the library appeared to be played out normally again 
plus I fixed the track length with the help of this post so we’re all happy campers. 
EDIT: for future reference, don’t change the base port from 443… you won’t be able to connect to your streaming service when you restart libretime services.