Finishing a screed floor

[Rustang]

Hi, new here and need a little help. Our builder has messed up our house this year but we are slowly getting there and sorting the last things out that they either didnt finish or did wrong. The un finished screed floor is what I'm looking to sort out next. We had an extension built and when they screeded the floor the doors werent in so the screed never reached the doors! We have a concrete base, damp course, insulation and screed up until approx 100mm away from the doors leaving a void about 200-300mm deep and 4metres wide running most of the way across the back of the house. So hows best to finish it off? Obvious concerns are that the void is deep and needs to be filled somehow so thats its stable for the last bit of screeding and then stable to walk on, Ive chipped some of the existing screed back because it raised up towards the doors and to also to help with a base for the rest of the screed to sit on which will be on part of the existing concrete slab. I noticed yesturday when clearing the void out ready to finish that it was a little damp so i will definitely need to put some damp course down but how, all sides of the void and on top before the screed or just on top of what ever base will be put in the void for the last bit of screed to sit on?