[Microbit-Python] Differences between BBC: microbit and pyboard

[hot.toast]

I'm getting confused over the differences between running micropython on 
the pyboard and on the BBC:microbit.

I have been playing with the  pyboard for a number of months and have a 
reasonable understanding of how to use micropython on that platform. But 
it is my current belief that the BBC:microbit does not ship with a 
python interpreter and that I have to roll my own using yotta and copy 
the resultant hex file to the microbit before i can get a REPL prompt .

1. Is that belief correct? (I don't seem to have to  load the firmware 
using DFU like on the pyboard)

2. I have successfully complied basic micropython to a .hex file using 
the web interface and got this running on my microbit, so I know the 
board is working correctly.

3. I have tried to follow the instructions on 
https://github.com/bbcmicrobit/micropython/README.md. using a Linux Mint 
17.3 system but when I get to "yt build" (or make all) I get the 
following error

[102/291] Building CXX object 
ym/ble-nrf51822/source/CMakeFiles/ble-nrf51822.dir/adsafdas/micropython-master/yotta_modules/ble-nrf51822/source/nRF5xn.cpp.o
ninja: build stopped: subcommand failed.
error: command ['ninja'] failed

Any ideas where to go next?