fireplace opening - lintel needed?

We had a "hole in the wall" gas fire fitted a week or so ago, which involved (the fitters) enlarging the builders opening (upwards).
Stripping the wallpaper today above the new fireplace surround, and some old plaster dropped away. I can see that the two bricks spanning across the 18" opening are not supported by a lintel or steel bar, and are drooping due to the mortar having crumbled. The row above are fine. The bricks will drop onto the fire if I do not do something.
Is it acceptable to not have a lintel or band? I can re-mortar the bricks in (less hassle than calling them back) but if there should be a lintel I will obviously get on the blower!
Any thoughts?
Tim Hardisty. Remove HAT before replying
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Personally, I'd have spanned the opening with at least a piece of angle iron of some sort to take the stress off the mortar and bricks, but that's just me.
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If these bricks are part of the flue, then you should recall the fitters to sort it out.
dg

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