wrote

I think this could be summed up as "bridging the cavity at a doorway".
On my build, there will be 65mm screed on top of the concrete slab,
and this needs to pass over the cavity to form a continuous floor
across to the door which is fitted in the outer leaf.
The inner blocks come up to the top of the slab, but not to top of
final floor level. The DPC from the walls is cut off at the door
opening. The DPM from the slab is actually long enough to cross the
cavity. But I was recommended to bridge the cavity with slate, then
screed on top of this across the cavity. The cavity closer will be
fitted before the screed and slate, and act as a sideways damp course
between the side of the brick opening and the screed.
I can see there are a few "corners" where damp could leak. I may paint
the area with liquid rubber just to be sure.
The earlier advice to have the inner leaf up to the floor level would
not work, since there would be no room for any screed on top, and any
self-levelling compound over screed and onto bricks / blocks would
forever cause problems. Basically, the screed must be seemless across
the cavity to the actual door frame / threshold.
Simon.

Thanks Simon
I think you have managed to clarify the situation somewhat here.
Can you explain what this cavity closer business is about please?
Is it the polystyrene cavity fill that Phil mentioned and showed in his
picture?
Can I do all this without removing the patio door? The above-slab screed
appears to go up to the face of the door sill rather than under it IYSWIM.
Also the patio door has a clearly visible black dpc under the sill.
Presumably the slate that you mentioned sits on top of the inner block and
across the cavity. In my case this would have to perch on the edge of the
outer block (assuming there is room left before the patio door frame).
Are you abandoning the dpm over the cavity completely - if not, where is it
going?
Are you talking about painting the slate and brick with rubber, or is this
over the top of the screed as a final sealer?

TIA

Phil