Darth,
if you look at the setup menu, depending on what unit you have, you will probably find a list of datums. Here you can set your unit to standard Lat and Long, or Decimal System for easy entry of co-ordinates, then mine has a long list of datums, you can get it to use OS mapping grids, american, tokyo and all other strange datums. I have experimented when in the car and dont seem to get much of a difference, so i cant proffess to be an expert, you could however try reading GColyers site. You will need a VERY BIG coffee mug.
