Finding UK Postcodes within a given distance

Question

In my asp.net website, I have two textboxes for the user TO ENTER A UK POSTCODE and THE DISTANCE IN MILES.
I want to find out all the postcodes within that range (within that distance).
At present, I am using ordinance survey method. I have a database with all postcodes and the corresponding North and East co-ordinates.
My logic seems to be not working. Could you please help me with SOME OTHER METHOD or  HELP ME TO USE a better logic in ordinance survey?