On thinking about this I remembered that I had an old "case" for my GS5. All I really needed to do was mount this safely and semi-permanently to the bike.
There was a "useful" gap in the clamp holding the handlebars on the head tube. I could use aluminium angle to slot in there.
The idea was to produce something like this:
I haven't really needed to mount a phone to bike.
It's only in more recent years (when I've no longer cycled) that use of mobile phones has been to pervasive as to be useful WHILST cycling, possibly since the app revolution in 2010?
This is what the solution looks like - to mount a GS5 to be used (viewed) while cycling.