Just bought one of these at the Kingston Park Tesco. They had loads of them - reduced from over £50 to £13.74!
Looks as if it's instore only (can't find it on the Tesco website).
This has been posted before at £17.99 from Play.com, but this price is even better.
Great sound for the money - battery or mains power. Will charge your phone if you're using the mains adaptor.
Seems to be compatible with most Sony Walkman phones.