Update:
This morning, having left the unit un-powered all night, I tried one last time to wake it up, but this time, recognising that it appeared to be in some kind of semi-permanent deep-sleep mode, I thought perhaps a “jolt” might shake the bastard up, and so I held down the power button and then plugged the power cord into the mains. Et voilà, it immediately switched on with associated mechanical “click” that is typically heard when waking the streamer from the app. All seems fine.
So what was the solution? Unfortunately I didn’t firstly try powering up without the power button switched on, but I had done that multiple times last evening and to no avail. Was it the lengthy time spent in the unpowered state? Possibly, although last night I had left it unpowered for multiple periods of > 5 mins and on one occasion about 1 h. Maybe it was a combination of both. Or maybe it was the pixies what did it. As to what caused the problem in the first place, I have no idea. Must have been pixies.