Fireplace lintel

Any help here would be much appreciated..
Currently opened up my fireplace and installed a new lintel:910mm opening with a 1200mm lintel inserted is this sufficient enough also found that I have another older lintel above it one more also in the bedroom fireplace just a diyer so any help would be great thanks

 

Taken back to builders opening..is 110mm sufficient overhang enough on the lintel on each side?as is only 960mm opening to fireplace I have read that 100mm would be okay for that opening as it is under 1000mm wide hope someone can help before I finish up boxing it in thanks

 

Thanks for getting bk jord86