After reading the following thread I took the bottom off my Nucleus to look at the circuit board lights.
They were all on so the device was clearly powered up. I reset the power light on the motherboard as advised in the thread and all seems fine.
My Nucleus is now running, but without an illuminated power button. Do I need to replace it, and if so, how do I go about this? The unit is out of warranty and I live in the UK.