I believe that to get the six digit postcode you require a special disc (as suggested above).

I think that the default navteq disc will only give you the first three digits. The disc I bought was about £30 and arrived next day. It also came with a free "latest" version of the OS.

This is the 2008 UK and Europe DVD high disc.

To get to the six digit postcode you need to go to "info on location" then "travel" then "UK postcodes". You then are prompted for the full code. It is very accurate. The speed camera locations are very accurate.

I have a MKIV unit running V31 of the OS.