"Greg" wrote
| 2. I figure I could use a structural steel beam to support the weight
| of the chimney in the attic ... and simply lay this across the joists
| with the brickwork resting upon it.

No ....

The attic joists are almost certainly sized only to take the weight of a
lath and plaster ceiling; even if they are sufficient for an ordinary floor
they may not be sufficient for supporting the weight of a ton of masonry.
The only way you can prove they are is by calculations, which the Building
Control will require to be signed by a Structural Engineer.

Owain