I've got some backing tracks (MP3) that I'd like to play along with but would like the option to change key and tempo - any software people can recommend to use on my iPad/iPhone to do this ?
I third the Amazing Slow Downer app. I have it on my iPad. It is simple to use and you can also slow down Spotify songs directly without having to bother with loading the music file yourself.
I like chord tracker by Yamaha. It's an app, lets you change key, change speed etc and even identifies the chords in the music you ask it to play.
Chord Tracker by Yamaha Corporation https://appsto.re/au/8qaj6.i