I'm inspecting firedoors where the gap at jamb is so tight you could barely fit a sheet of paper in. 

The data sheet and the fire test report only mention maximum allowable gaps. BS8214 (9.5.2) says that a typical gap should be 2 mm to 4mm.

 

Is there a minimum allowable gap? Should I accept these doors?

Thanks.

 

If the door still closes ok then it is fine. 
 

The problem with tight gaps is that the door could bind on the frame and fail to self-close correctly.  Always reference the BS 8214 guidance in your report.