We came down the following morning to check the slab and find that it has hardened sufficiently to go down for a closer inspection.
Putting the laser level on points across the slab make me feel better about the exploits the previous day, as we are generally only 2-3mm out across the entire slab. At the far end we are between 5 and 10mm out, but this is where we dragged the concrete across the length of the pool to ensure that we hand enough for the drain end.
Having worked it out, this different is equivalent to 0.1% slope, which I can live with. It can always be made up in the latter stages with shuttering on the top course.
Now we are travelling back to the UK for a few weeks for my daughter to start University and I can get on with planning and ordering the blocks (bloc de coffrage) for the walls.
I hope to get back out in October to start on the wall, but that will also depend upon how COVID-19 is and any return to lockdown in either the UK or France.