To read M-Bus on Pi serial port you need to swap serials...
To disable onboard Pi3 Bluetooth and restore UART0/ttyAMA0 over GPIOs 14 & 15 modify:
sudo nano /boot/config.txt
Add to the end of the file
dtoverlay=pi3-disable-bt
We also need to run to stop BT modem trying to use UART
sudo systemctl disable hciuart
mbus-serial-scan-secondary -b 2400 /dev/ttyAMA0
... Found a device on secondary address 48420932EE212004 [using address mask 4FFFFFFFFFFFFFFF]
mbus-serial-request-data -b 2400 /dev/ttyAMA0 48420932EE212004