Best price I've found for this elsewhere was well above £20. Play.com have this bluetooth device, which clips onto the sunvisor of your car, for just £14.99 with free delivery. This is great as it's more portable that most kits. Just unclip, and move it to a different car!! It provides a much better sound quality, too.
The Wireless Simple Bluetooth Car Kit provides a simple top quality option for your hands free needs. By simply attaching the unit to the sun visor of your car you get much clearer conversations as the microphone is closer to your mouth than other dash board mounted kits. The all round sound quality of this car kit is excellent because of the integrated Digital Signal Processor (DSP) which gives built in noise canceling technology and echo suppression.
A high powered Li-Ion battery provides the Wireless Simple Bluetooth Car Kit with an approximately 33 days standby time so you will never miss a call again! Another enticing feature is that a headset connection is provided incase you want to make any private calls.