I don't know what it was showing when it was failing to charge the other night because it was the middle of the night. But no I haven't noticed any indication that is isn't charging, whenever I have looked at the phone when charging it has always had either the "charging" or "plugged in but fully charged" symbols.

Also, it can't be down to that single charger, because two of the days that it failed by lunchtime it also spent over an hour plugged into the car charger, which would normally be enough to take the charge to nearly 100% even if it was low.