buy some grit sand ( screed sand) mix with water and cement ( 3 or 4 parts sand, 1 cement) but only enough water so that a handful of the mix only just holds together when squeezed. start by laying perimeter screeds starting just in from the door way all the way round the room and back to the door. then just fill in. you will need a plastic float to "work the fat", a 1200mm level and a floor float to finish. floor screeds tend to crack after a while, some recommend thin mesh, others a fibre additive, others
couldn't care less as its being tiled/ carpeted/ laminated anyway.
p.s.25mm is a little on the thin side for regular screed maybe a granite chippings mix could be the answer? 2parts chippings, 1 part screed sand, 1 cement.
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