Excellent news from my side.
Michael: When you said 'Look in C:\ProgramData, I could not find your quoted folder.
For me C:\ProgramData did not exist.
I assumed you meant C:\Program Files (x86)\Mission Planner\srtm
However after some frustration, I decided to turn on - show Hidden folders,
and look again...........and there it was; the full path C:\ProgramData\Mission Planner\srtm.
I inserted my GeoTiff into this srtm folder, and all is good.