I have removed an old backboiler from our lounge fireplace and
surrounding filler bricks so now all I have is the original builders
opening. The arch has a supporting steel bar underneath it.

I need to reduce the original openings height and width and estimate I
will need two rows of bricks to reduce the height to the required size.

What is the best way of supporting these bricks? Should I buy a
standard concrete lintel and cut it to size (could be difficult ?) or a
steel one (again difficult?). This must be a fairly common scenario so
how is it normally done?