Not to worry, got to the bottom of it.
Turns out even though the device is internal to the case, it is still plugged in via a USB port.
I have a bug at the moment with my PC where USB devices left plugged in when the PC restarts do not always make themselves available for access (until I unplug them and replug them in again).
What had thrown me off was that the power light had been on at the front, so I didn't think it was a dodgy connection. Just replugged the cable in the case and it is working again.
Grr....