how does the water completely go into the drain, or will you get water sitting around the drain at the bottom of the slope as there is no angle towards the drain
You are right to notice that there is a flat area around the drain and that some very little amount of water will just pool around it, will not flow into the channel.
However this is not a problem as it will be a minimal amount of liquid, 1-2 mm deep. Anything more than that will be drained. Remaining water can easily evaporate.
There is now way it would move away from the flat rectangular area, because it's sitting at the lowest point.
I will be happy to assist if you have any questions.
David 020 7193 0953