We had a standard 22" square open fire with a clay fireplace. We've now
removed this with precision hammers, and are in the process of getting a
woodburner installed.

I want to put something in behind the woodburner, for two reasons: firstly,
to tidy up the appearance by hiding the rather unsightly brickwork, and
secondly to reflect heat back out.

I was thinking stainless - basically about 2' x 4', with the sides folded
round to fit the hole.

I've been quoted 90 quid for heavy gauge mild steel, painted in black stove
enamel, and 180 quid for stainless (apparently it will need to be good
quality to stand the heat, which makes sense).

I'm happy enough with black painted steel from a cosmetic point of view, but
I'm wondering about heat reflection. My memories of physics lessons
suggest that if the front is painted but the back is left unpainted, it
will tend to absorb heat and re-radiate it back into the room. Is this
correct?