After some web research and discussions with various people (Gavin who poured our slab, Rusty at the hardware, the people at Bondall) I realised that I needed to prepare the raw rough cement with Bondcrete.
This needs 24 hours to dry - so if I intended to pour a screed on Saturday morning then I really needed to get out to the house on Thursday night and apply it to give it enough time.
I gave the bathroom floor a good sweeping with a small brush, followed by a thorough vacuuming.
Then I applied Bondcrete diluted 1 to 4. I used a big, cheap old paint brush and it really took a lot of effort to penetrate in to every nook and cranny in the rough surface.
I took me 3 hours to prepare an area 2m x 3.5m - I hate to think how long the laundry area (8m x 3.5m) is going to take.