Crack in stone fireplace Lintel

[armchairsupervisor]

Hi All,
I have recently started to knock out my fireplace, have taken it back to the top most lintel and noticed a large crack running through it, got a bit panicked and put up a couple of brick supports to slow my heart down a little. On closer inspection I can see soot over the lintel and inside the crack itself so I presume it has been there for a while (the fireplace has been unused for at least 12 years). The crack appears to be all the way through the stone, the dimensions are 68 inches long, 11 inches wide and 5 inches deep into the fireplace. The crack is 5mm wide and hasn't changed in 24 hours.
From what i can find out easiest solution would be to fix steel plate to the underside/back
Here are a couple of photos, any advice gratefully received

Stone lintel exposed

• armchairsupervisor
• 16 Sep 2010
• 2

Lintel crack close up

• armchairsupervisor
• 16 Sep 2010
• 2

Many Thanks

 

[merlin50]

so you are starting from the bottom and working up

 

[armchairsupervisor]

I should of said opening up my fireplace, I have taken out two smaller fireplaces and got it back to the largest opening, hence the lintel that is in the photo, I intend to put a log burner in.
I would like opinions on whether I need additional support to this lintel because of the crack in it. In other threads i have seen that many cracked lintels are OK to leave as they are in no danger of falling due to the weight of bricks above the lintel that is embedded into the side walls, giving a counter weight/lever effect.