My brother is having a new fd30s installed to his flat.

The original survey measured the height of the existing opening but unknown to them the plasterboard soffit hid the fact that the lintel was higher than the frame by approx 50mm, the door is now  made.

Can the 50mm gap fe infilled with solid timber/rockwoo firepro etc. and sealed and still comply

Yes it is possible to resolve the issue without ordering a different size door.  But the method used depends on whether the door is composite or timber. 

If its composite the wall should be extended to the same (or greater) specification as existing. If its a timber door the wall aperature may be lined or the door frame head thickness extended using the same materials as the new door frame. 

Linings / additions should be full width and continuous in length with no gaps and/or voids. Joints should be tight and fire rated linear gap sealing to be in accordance with BS 8214: 2016 requirements for timber doors and manufactuers certification for composite.  

Would not the door manufacturer need to provide approval including test certification for this particular situation

Check the scope of the available evidence of fire resistance performance for the particular door / door-set.  If, having done that, you believe that the necessary works required to enable correct installation might compromise the performance of the door, then its apropriate to check with the door manufacturer accordingly.  

Ttlhe company is fitting a hardwood timber to the lintel sealed around with blue 60