Harry Bloomfield explained :
The horizontal fill-in strip along the bottom (which a lighter double skinned
material), sits on the bottom lip of the L shaped vertical as support for its
outer edge and the correct way to support the edge nearest the wall is using
either a timber batten or a U shaped plastic channel fixed to the wall -
however I notice that most/all of the nearby houses which have had this done,
seem to have had the wall edge simply stuck with silicon. This strip has to
be cut along its length to fit the gap.

I would guess the wall edge has been simply supported with nails temporarily,
whilst the silicon is allowed to set, then the nails removed. I'm not sure I
would be happy doing this?

I have an idea which would solve this, with no need for a timber batten
or U channel along the wall...

The double walls are joined by a thin skin every 1" or so across their
width. If I were to drill, plug and fix some screws or nails along the
wall and at the centre line of where the horizontal panel should meet
the wall, one fixing every foot or so leaving the fixing stuck out a
way - and I also cut a hole in the thin skin to match up with the
fixings, the panel would hopefully be able to be dropped straight onto
these, then the front edge dropped onto the lip vertical.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk