Okay, hang on. I'm confused with this sitting thing (and structure in general).
I can tell you how the Swedish CAA does it -- of course the UK CAA may interpret the EASA rules differently.
If you want to do all units, you have all of 6 "sittings" to complete them. BUT as you are not allowed to retake a unit in the same sitting, you should try to do all units in as few sittings as possible and not spread them out.
If you fail one unit, you have to do that unit in another sitting. You don't have to retake the units you have already passed unless you run out of sittings OR have failed four units (I'm not 100% certain about the number here) -- then you have failed the entire exam and have to retake all units after a new approval from your ATO.
E.g.: Suppose you spread out the units and take 2 in sittings 1, 3 and 5 and 1 in sittings 2, 4 and 6. Suppose that you pass all of them except the last. You would then have spent all your sittings and would have to retake
all units but only after a new approval from your ATO...
