Well, testing the first bit again...washed in another 3 bags of sand, I think we're close to having filled all the gaps.
Now waiting for the water to drain away and will try some compacting again after lunch.
let some water into the tank as well, to counterweight from pressure below the tank and to check if the water stays now.
If all ok, then I'm going to put in another 20 cm, slowly bringing it up.
Very small leaks will settle with dirt anyway, so hope to have it all finished after today.