I decided that my maps from 2009 were in need of an update
So I found the required files floating around on the internet, accquired them and put them onto a SD Card.
Then went to delete the MAP and POI folders on the F900BT
Then copied to new files on
Was a bit worried that they might not work, so kept a backup just in case, but I needn't have worried
All working
Now up to date with TomTom 2011 Q4 maps
Only thing is that the F900BT has limited space on it's internal HDD and won't be able to fit the entire catalog of EU maps, so was just a matter of picking which ones were essential (like consider whether you will actually drive in Latvia, Finland or Norway, etc.)