To be honest I Think your wasting your time trying to waterproof the patio, you will always get abit of water under there and it should just have some weep vents so it can run straight out, also I would have though that any water that goes between the flags will go straight down between the blocks in the block and beam and into the ground.
I would say that you have some dodgy bricks, but its worth making sure that there are weep-vents to let the water out, replacing the bricks is not a big job
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