I'm renovating my rental property and have been told by the council that I need to ensure that all my fire doors meet the floor or leave no bigger a gap then X amount. what can I install on my doors to make sure that there is an appropriate seal? all the doors in question are internal fire doors.

You need to speak to the building control officer but he may accept a hardwood lipping to the bottom of the door within 3mm +/- 1mm of the normal floor level with a similar gaps on all other gaps.