A little more progress has been made purchasing parts for the board. A trip to toolstation means I now have a 1m length of M12 threaded bar which will be cut to length for the axle. I've also purchased various nuts, bolts and washers to secure everything together.

Also I felt like doing a quick mockup to get a feel for what the final board will look like:

Finally I managed to do some more testing of the motor controller. I have now proven to myself that I can easily send commands to the motor controller from an Arduino via the UART (serial) interface. I'm not sure whether I will use an actual Arduino in the final version however. There is only one serial interface available which is connected to the the USB<->serial interface. Any serial commands sent from the PC will also arrive at the speed controller. I do have some bare ATmega328P microcontrollers lying around, so I might use one of those and program it via ICSP instead. Alternatively the speed controller has an emergency stop pin which, when pulled low, disables the motors.