I am currently renovating my kitchen and am looking for some advice in how to screed over a cavity. A section of the old screed layer has broken up around the edge of the patio door threshold. At the moment, there is a dpm under the patio door overlapping the cavity and meeting the main dpc which comes up from the slab and wraps around the inner bricks (see pics). My question is, how do I go about screeding over the cavity? I’ve seen suggestions of placing a sheet of dpm in the cavity to form a trough (lining both the outer and inner bricks), filling with celotex or similar and then screeding over. Is this the best option? I just don’t want to be ripping the new flooring up after 2 years. Thanks in advance