The moral to the story.
Swap your batteries, with your friend's , don't tell him and everyone will be happy.
Sorry Bad Humor.
Ian I don't know. Obviously it doesn't make sense. Just for the hell of it see if your batteries have many flashes in them. Flash 20-30 times. See if it holds up. Clean the contacts. It must be something simple that you're overlooking.
Not much help sorry.