all sittings (up to six of them allowed), should be done within 10 days according to UK CAA rules.
No....
Each sitting is a period of 10 days starting from the first attempt on a unit in that sitting!
The AMC to FCL.025 states:
(e) ‘Sitting’: a period of time established by the competent authority within which a candidate can take an examination. This period should not exceed 10 consecutive days. Only one attempt at each examination paper is allowed in one sitting.
So this means that if you fail a unit on the first day of a sitting, then you have to wait (in the UK) 10 days until you can make another attempt on that unit. If you fail the unit on the last day of a sitting, you can make another attempt on the next day!
To make things even more fun, the Swedish CAA interprets a "day" to mean a working day, so in Sweden a "sitting" is 14 calendar days!
This is all sheer lunacy!
PS. A correction to what I wrote earlier:
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.
It should not read "failed four units", but rather "failed four attempts on the same unit".