looking in the relevant folders I find several files listed:

building:

United Kingdom_NQ_2011.Q3_111114.3dc
United_Kingdom_83_NNG_2009.12_100224.3dl
United_Kingdom_83_TA_Basic_2009.12_100325.3dc
UnitedKingdom.3dc
UnitedKingdom.3dl

poi:

United Kingdom_NQ_2011.Q4_120127.poi
United Kingdom_TomTom_Premium_2011.12_120118.poi
UnitedKingdom.poi

Do these files compliment each other i.e. do they all get loaded and some contain information others dont so that they create a better overall picture or do does only one get loaded and if so which one and is it the best?

In general and as far as I know, only the latest files are loaded. That is certainly the case for map files (.fbl). I am not 100% sure how .3dc (buildings) and .3dl (landscape) files are handled. I suggest to delete the oldest ones.
However, in case of POI files, but from a different maker, thus different dates, they are both loaded and the information can be displayed double where both files have the same info. Where they have not, they are complementary. This can be very locally or all over the place. I suppose all 3 files are loaded here.

archercj wrote:looking in the relevant folders I find several files listed:

building:

United Kingdom_NQ_2011.Q3_111114.3dc
United_Kingdom_83_NNG_2009.12_100224.3dl
United_Kingdom_83_TA_Basic_2009.12_100325.3dc
UnitedKingdom.3dc
UnitedKingdom.3dl

poi:

United Kingdom_NQ_2011.Q4_120127.poi
United Kingdom_TomTom_Premium_2011.12_120118.poi
UnitedKingdom.poi

Do these files compliment each other i.e. do they all get loaded and some contain information others dont so that they create a better overall picture or do does only one get loaded and if so which one and is it the best?

Let me just add this. ALL content goes into your device's memory. That's why we suggest that you delete the dems and buildings when someone's program crashes. 3dc and 3dl files are buildings [in major metropolitan areas ONLY, not everywhere] and are different from POIs [points of interest].