Put it low
down, but not as low as the one I've just had to reposition when
refurbing a room recently: the old grille was completely occluded by
soot and debris which had fallen down the chimney!


.... i was going to place them just above skirting board level.

If you can ventilate the stack from the outside, or even from under a
suspended floor, then it may be a different matter,

....no can do...exterior wall ajoins neighbours garage

but otherwise I'd
just consider capping the relevant pot with a vented cap.

....i had thought of taking no action other than capping. However, many
houses I've seen where the old fireplace has been bricked up seem to
have employed an air brick at some point.

You just want to put them where the old fireplace would have been,
wherever that was. Not necessarily central on the chimney breast if
there are several chimneys and back to back like yours.

I'm not sure what you mean by back to back? I was hoping to avoid
placing an air brick on the first floor. Also, the problem still
remains with the air brick in the attic; potentially there are two
channels behind it - would that mean two air bricks are required and
then a degree of guess work when fitting to ensure that they each
perforate one of the channels behind?