The CAA is very good at confusing the issue, which is why I always prefer to go to the source material.
The
actual requirement (FCL.025(b)(2)) states:
Unless otherwise determined in this Part, an applicant has successfully completed the required theoretical knowledge examination for the appropriate pilot licence when he/she has passed all the required examination papers within a period of 18 months counted from the end of the calendar month when the applicant first attempted an examination.
The clock starts ticking at the end of the month in which you pass the first examination and if you haven't passed all of the others within the following 18 months, all of the passes are invalidated and you have to start again. You cannot reset the clock within the 18 month period.