For some reason the main box in the lounge has rebooted on first switching on this morning. Thought it might be an update but the other box came on straight away.
The affected box went through the Welcome, Bienvenue etc, etc screen then went blank with light turning blue. Switched on again and it went into reboot mode again. This time I waited until screen went blank and before indicator light had gone from yellow to blue I pressed the power button again and this time got it to fire up correctly.