"Don't let wires restrict your movement when you are enjoying your favourite games, free yourself with the Wireless Nunchuk Adaptor. Compatible with all games that require the Wii Nunchuk the Wireless Nunchuk Adaptor fits comfortably in either left or right hand. Powered via 2 x AAA batteries (included), provides up to 30 hours playtime. "
Reviews suggest it may be hit or miss, and it doesn't follow the typical nunchuk shape, but for a tenner, hope it helps someone.
EDIT: Thanks to brilly for pointing out that this actually an adaptor.