Ring Powerring E:Can Plus Power Inverter (RINVC150)
Instore at Sainsburys for £7.50.
I know this was posted a month ago, but has since expired.
It seems Sainsburys have got more of these in stock.
£7.50 from £29.99 !! (£24 @ Tesco - http://direct.tesco.com/q/R.201-4442.aspx)
It is the 150w version, and includes a usb charging socket.