I've now abandoned the idea of making my own stepper motor drivers and gone for the ready mage Chinese driver instead: AC / DC Universal M860H 7A Two Phase Hybrid Stepper Motor Driver For CNC

The driver requires 5V signals to run it and I've set out the PCB above with a whole load of 5V Arduino Nanos networked with a master Arduino Due (or MEGA) with data transmitted via I2C. I'm guessing that each Nano can control 2 motors with accelerations as, from memory, there are 4 timers available on each board. Could be wrong on several accounts, but I've incorporated enough redundancy and extra features such as logic level shifting to cobble together something from my PCB that will almost certainly work. I'm giving it odds of 99%.