result
I formatted the unit only and then copied the back up that i had done many moons ago back to the unit.
I now have full postcode back on both standard 520 and truck map.
I had recently updatde the 520 (after removing the sd card) via tomtom home ,so it seems that may have caused the problem. now the 520 is back to earlier stage all is fine.
Thanks to tendriver for his help and for his advice to back up (made on another forum ages ago) any stuff before you alter it,
i think this issue is now solved for me:)