Use the link.
If you have a TOMTOM you can register for a free £10 voucher - which I then went on to buy Homer's voice with. Then I registered with 2 other email addresses to download Night Rider and Billy Connolly apps with.
Follow the link, register your details and you will be sent a £10 voucher by email to use straight away on the TOMTOM shop.
Just enter the voucher code into the left hand box when looking at your trolley.
Your email receipt will have an attachment - download it.
Once completed, go to TomTom Home on your computer, make sure you are logged on with the same email address that you bought the voice with and then connect your device. It should automatically update and itself and store in your voice selections.
If you've already registered just sign up with a new email address to take advantage of this. You might have to adapt the email attached to your TomTom Home to download it.
Homer is available through this linkhttp://www.tomtom.com/page/simpsons?Lid=1&selector=true
Darth Vader voice is available athttp://starwars.tomtom.com/voices/index-starwars.php?Lid=1
Nightrider is available at the the following linkhttp://www.tomtom.com/page/kitt?Lid=1&selector=true
*thanks to saintscouple
Billy Connolly is available athttp://www.tomtom.com/page/billyconnolly