Wilfried,
UK postcodes are split into a hierarchical system. Sector, area, etc. I'm always forgetting which is which
Eg. "CB3 0DS".
"CB" means it is the Cambridge area (that alone makes it more natural for humans than the US zipcode system). The CB area is pretty large, and is split into smaller areas. I think central Cambridge is split into at least three.
That is then split even further to "sector". Eg. "CB3 0??"
The last two letters designate the final postcode in the sector. The Post Office claim an average of about 12 addresses per postcode, although many offices and larger locations have a dedicated postcode. Eg. CB3 0DS is Churchill College, Cambridge (and no other addresses).
I can't really help the original poster - I've also noticed that MapPoint only really works with postcode sectors. Although it sometimes recognises full postcodes correctly, it appears to be for location finding only.
Richard