A very late update... in my case it was a faulty controller PCB. I took my bike to the dealer (under warranty). He suspected the switch or cables but was surprised when it was eventually traced to the controller PCB. Apparently the first he has needed to replace from 100s of bikes sold.
I have experienced exact same symptom as the OP (who seems to have come and gone). Out for an easy dry ride, battery at 80%+, stopped for 5 minutes to look at the view. Bike wouldn't switch back on :-(
I have checked all connections are clean and tight (everything looked clean before I started...