Thanks for sharing this Colin.
For years I've been using an app on an Android phone to listen to turn by turn instructions for the routes I plotted. Just one earpiece and with the screen turned off.
I've always used Track Navigator. It only guides along plotted routes, nothing else. It's very simple, it's free and works really well. I export .gpx routes from Plotaroute.
However, there is also another app which I am surprised I've only just stumbled across. It's called BIKE TRACKER. It's only on Android, it's free, it guides along plotted routes BUT it can also output information of your choice along the way. It focuses on cycling but there's no reason not to use it for other activities. You need to use .tcx files from Plotaroute
This is a very flexible app with a lot of help text and it's all put together by the excellent Steve Punter of PC Ability.
You need to know the following as there is more than one app called Bike Tracker ...
Download from https://play.google.com/store/apps/details?id=com.pcability.biketracker&hl=en_US
Support Group https://groups.google.com/forum/#!forum/bike-tracker-support-group
Finally, to make it easier for anyone to get started, I created a Quick Start Guide and pulled all the help text together to make it easier for me to read - and pasted it all on the Bike Tracker Support Group. Go straight to the help text here ...