Glad its not just me this is happening to. The battery drained for the second time last night. I thought it might be that I hadn't fully charged the battery after the first occurance about a week ago, but this might not be case now that I've read this thread.
I've performed a full Device Shutdown, checked that the Wi-Fi is off (its usual condition) and am recharging the battery as I type. Will wait to see what happens over the next week or so.