...so, I finally found out how to stop random music popping up on my sat nav whilst riding. I hope...
There is a feature called auto play that you can turn off, but my iPhone SE doesn't seem to have this feature so I can't blame that.
So, I don't know what causes random music or even the random appearance of the music player on my sat nav screen but it appears that there is a way to stop it. Simply delete the music app. But that also loses all your music, which is a bummer because I love playing music through my Bose speaker when relaxing in the summer house, or working under the car or decorating the ceiling, just NOT while in control of a lethal machine at speed.
However, there is another way. You can 'offload' the music app (and other apps I guess) which stops it working but keeps all the music you have on your device.
Settings > General > IPhone Storage > Music
You have two options, Offload App or Delete App.
Once offloaded the music ghost should never appear again. When I want to play music, I can go back and the Offload App has turned to Reinstall App and clicking on it does just that and takes no time at all.
Alternatively, simply click on the music icon and it will automatically reload the app and all your music is still there.
Simples! And hopefully, blissful silence apart from the sound of the music of my tyres, engine, the wind and the nagging of the sat nav.
I'll let you know if it works.