Still no joy: I had hoped that after another factory reset (done in accordance with precise instructions from Karl Harrison at Humax) following Wednesday's Freeview retune my issue would be resolved. Oddly enough, although the Humax box didn't reconnect automatically after the first time it had been on standby, it did do so after the second and third times. But my joy was premature, as I was back to square one the next time, on the next day: "Network disconnected", and go into Settings to reconnect (all the details of my network are displayed, but the software doesn't apply them and requires a manual "Apply" as before).
Meanwhile I have a new laptop on my wireless network and it connects instantly every time...
I give up.