Hi,
You've probably already figured something out, but anyway, you need some service to translate the users postcode to latitude and longitude and then use some math to calculate the distance. Of course you don't have to reinvent the wheel, just find suitable parts and put them together in your custom filter control. Except searching for solutions, a quick guide could be the "marked as answer" post
here at stackoverflow.com.
Hope this will be helpful.
Regards,
Zdenek