A perusal of
Safety sense leaflet 20 might be beneficial.
If your FPL is for a future date, make sure that the date is entered clearly in the remarks section, item 18 (eg RMK/DATE OF FLIGHT 27 Sept
that flightplans are to be filed 24 hours BEFORE departure
Where does it say that? I thought flight plans could be filed up to 1 hour before departure?