does anyone know how to check a postcode is correct?

i've bene looking for one that puts the building about a mile away from wehre it actualy is, so i need to kow what postcodes are going to be in that general location rather than just the specific postcode, so is there was map of some decprition that tells you which buildings amd areas are covered by what postcode?

thanks

 

http://www.royalmail.com/portal/rm/p...r?catId=400145

 

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