Quick update for people asking me what phones it works with, whether it works with an iPhone / iPad and potential PC based software.

PHONES that definitely work using the (free) FluidSynth app for Android phones that I originally intended it work with:
My old MOTO G4 works OK.
Also from currently available new budget phones: 
STK One Max    
Nokia 2.2

Apps that work:
I have it working, as I originally designed it, with: 
Fluidsynth Midi app (Free from Google Play store) on an Android phone.

Others also have it working in these ways although I have not tried these myself yet:
a) bismark bs-16i app on an iphone, [NOTE: You have to use the Apple camera adapter which is about GBP35], cheap ebay versions may not work.
b) Also, Reaper with the TX16Wx software sampler plug on a PC.
c) Also apparently works with a device called an Organelle M.       https://www.critterandguitari.com/organelle