This is a warning to folks out in Northern Ireland or who need their sat nav out there...
I discovered rather to late (getting off the ferry in Belfast) that BT postcodes do not work on the latest 925 zip/925_5439 UK & Ireland combination - possibly other Europe maps including GB&I too. It is a known fault that TomTom are aware of (there are various posts on this on TomTom's own forums) but they do not seem to have fully fixed the problem yet.
So be warned - if you need your sat nav in Northern Ireland stick with the 920 maps which work fine.