Andy,
as said above, if you are putting a fire back in then use a
lintel. These even have the correct angle for the draughting made into
them.

When I moved into my first house, the EXPERTS, had manufactured
their own lintel as it was a through the wall type fire place serving
two rooms. They made the angle so shallow that 49% of the smoke went
into one room, 49% went into the other and only 2% went up the chimney.
Luckily the seller was able to with-hold a final payment to said
EXPERTS and gave it to me to get the job done correctly.

The internal angle for correct draughting is about 43 degrees.
I was able to find it in a book in the library. Probably on the Net
now.

Chris.