Michael Chare wrote:
On the downstairs windows, my house has a row of vertical bricks above the
windows, rather than an identifiable lintel! I am wondering how much the bricks
are supported by the metal window frames, which will be quite strong as there
are vertical bars.

Are you sure the bricks are vertical? More likeley they will be a very
shallow arch. Look for the spacing between the vertical joints
increasing towards the top, or for the gap between the arch and the
course above decreasing towards the middle.

The interior walls on my house, c. 1900, use arches for doors with a
rise of little more than an inch over the door width (The exterior is
stone). The brickwork was quite stable with the standards removed.

Is this a common arrangement? Do I have to be particularly careful about any
replacement windows that I might want to fit?

I think it's pretty common. If the bricks and mortar are in good
condition and it is indeed an arch then it shouldn't put any load on
the window.