Find the Fire by Bradley Dean uses http://nuffcumptin.com/projects/redfearn/redfearn.php to translate the grid reference to LonLat co-ordinates.
As CFA pager coordinates are only 6-digits they only work near to the brigade they are sent to (because they only include part of the actual Easting and Northing values). This system attempts to guess the missing Easting and Northing components based on the location of the fire station. This means that you cannot enter the pager reference for an incident out of the area for the fire station you have configured..
As fire stations do not exist in the middle of their truncated Easting/Northing grid, and as they can be quite close to the edge of their box (for example Woodend firestation is at pager coordinate 808625) a pager co-ordinate may overlap the edge of the truncated box. For example a page to Romsey might look like 005630 - but if that coordinate was taken to be in the same 100,000 wide grid that the Woodend Station was in the page would refer to a location far to the west of Woodend (far outside the usual paging area). To handle this situation a pager coordinate that appears to be more than 500(00) away from the station in either Easting or Northing is adjusted to be in the next box.