I decided that given the concrete is relatively fluid (S4) then this could cause a problem between the main walls and the small wall in the middle of the pool. The small wall is to act as a separation between the shallow end and the slope and is made up of three courses of blocks.
However, I fear that the pressure of concrete from the adjacent eight courses could cause concrete to backup and overflow from the lower and exposed three courses.
Therefore, I went to the DIY (Brico) store and got four bags of ready mix concrete. I mixed these up and poured them into the two end of the small wall. This way, it should stop any concrete from the connecting eight high wall to flow into these blocks and overspill.
I also got a bag of ready mix mortar and filled around some of whole around the re-bar projecting out into the pool and also any other small gaps that might cause an issue when pouring the main walls.