Solved - user error :)
Inside the Raspberry Pi setup (file /MySensors/examples_linux/mysgw.cpp), the inclusion mode define must be added:
#define MY_INCLUSION_MODE_FEATURE
This is present in the Serial & Ethernet gateway example ino files, just missing from the Raspberry Pi version as it is on GitHub.

With this added, the response to the StartInclusion mode request is then received & everything is fine:

• TX(fromController) 0;0;3;0;5;1
• RX(fromRaspberryPiGateway) 0;255;3;0;5;1

On the plus side, by working this out I'm a lot more familiar with the workings of MySensors!

Cheers!