Hi all - bit long this one sorry (no piccys available)

I have a damp problem which appears to be coming from the strip of floor
just inside my patio doors.
Originally this damp was believed to be due to a leaking pipe, so a channel
to the room side of this strip has been excavated to show the pipes.

To clarify - considering the view from the inside out there is:

Floor slab
Channel in floor with pipes in right on edge of slab
Edge of floor dpm
Inner leaf of block work with concrete capping to level with existing floor
Inside face of patio door

The damp proof membrane for the original main slab ties into the wall dpc at
top of main slab level.
This means that any dpc/membrane below the strip of floor immediately inside
the patio door (above the inner block leaf) can only have a membrane below
the level of the wall dpc alongside it. I should point out that this patio
door is not an original feature of the property and is effectively a window
extended to ground level IYSWIM, although a patio door has been there for 20
years or so.

So I guess the questions are to do with the way this area should be made up.
If this door was installed from scratch, the brickwork would be cut down
below the window but what then?

1. Is the exposed cavity filled with anything down to foundation level
(concrete)?
2. How is the dpm for the room slab connected to dpc under this strip of
added "floor"?
3. Should the floor strip be just concrete, cast over dpc?
4. How is the concrete strip supported over the cavity, if the cavity isn't
filled?

Having poked about a bit, the damp appears to be coming from the area
outside the main floor dpm, under this strip of floor.
Under the concrete strip there seems to be loose sandy stuff. I am looking
at a small area alongside the pipe channel about 600 (2') long. Although
there is some dpm under the strip it is clearly damaged in parts.
The insurance company put some driers in over the past fortnight, but within
an hour or two of them being turned off, damp came back to the sandy area
beneath the floor strip.
If there is no "active" water source (i e. if it were down to ground water
only) would damp reappear that quickly?

Anyone who's read this far deserves a medal - any help or advice
appreciated.

TIA

Phil