The issue seems to be with the Mega Mini board.
I managed to remove the Adafruit RFM69HW daughter board from the verroboard, and wired this to a Nano, and a Teensy 3.2. Both worked with the same PSU.
The USB converter however is still giving garbage , no matter which of the three sources above.
So I am in the belated process of porting code to the teensy as the target.
Many thanks for the help to you all.
Now, if I can only sort out my sensebender gateway problem....
@skywatch Just to add to your points on 254 max children, definitions, presentation, etc., which may help @mrmuszynski at some stage.
I've an array in a sketch with 14 DS18B20 addresses, all paged in turn using the one wire protocol on a single pin, so it is entirely possible to hit the 254 children limit before looking at I2C devices.
The beauty of the DS18B20 array is individual chips are "defined" sequentially, hence making presentation and fetching values available to a FOR loop, which makes for a highly compact sketch.