An hour or so later when I took it off the screen the phone is dead. All buttons unresponsive, tried different chargers, then took it to a shopping centre repair place who tested the battery and said it had charge in it. The LG charger still works perfectly with my S3 so it can't be the charger.
So I'm gutted as I've just returned from 2 weeks holiday and all my photos are on it. I tried plugging it in to a USB port on my PC but no joy.