I am trying to build a fireplace, to put a wood burner in.

I intend to use clay flue liners. I intend to build a double skined
chimney, initally from 7N blocks, but then into something that looks
good once it comes out the roof.

So far I am up to lintel level, I have my lintels cut to size.

I have worked out how to hold my bottom flue liner in place, while I
go up a level, but I have some puzzles ........

What holds the bottom flue liner in place? is it simply the fact that
it has motar on the sides. If this is the case, then I assume I have
to pack this in quite well.

Whats finishes the top of the opening ? I am thinking of putting a
cement sheet in (like MDF but made of cement), all cut to shape, with
a hole for the flue liner to protrud through.

Once I have the first flue liner in, am I supposed to fill between
the flue and the brickwork with vermiculite or a "week lime mortar" ?
If the liners are not held on the way up, then they all rest on the
bottom one.

Do I seal between the flue liners with fire cement ?

Thanks
Rick