Keep the Mars bar in the fridge for at least 4 hours before cooking, otherwise the batter doesn't stick properly and cleaning melted mars bar out of a deep-fat fryer is not fun!
The variations on this recipe are endless, I've successfully managed the following * Creme Egg (watch the centre when you bite into it - HOT) * Bounty * Milky Way * Curly Wurly * Fudge Bar (not great to be honest)
With a big enough fryer you can even batter a small pizza