Right... breakthrough happened today... (As in, I realised I have special needs)
When I copied the .mobi filed to the /kindle folder of my SD card yesterday, I immediately tried to view the files on Kindle, and nothing was appearing, not even my Amazon downloads, until highlighted and re-downloaded from the archive...
Sine disconnected from the USB lead, and my phone re-scanning the SD card, both my test .Mobi files appear and are readable in Kindle App! The text formatting is a little bolder throughout, but it's more than readable.
I'm going to try a batch conversion bunch of PDF's I converted to .mobi later, and will see if it works also.