Schedule 12 Part II Para 7(2) of the LGA 1972 states that:
"In a year which is a year of ordinary elections of parish councillors, the annual meeting of a parish council shall be held on, or within fourteen days after, the day on which the councillors elected at that election take office, and in any other year the annual meeting shall be held on such day in May as the parish council may determine."
Section 16 Para 3 includes:
"...the whole number of parish councillors shall retire together in every ordinary year of election of such councillors on the fourth day after the ordinary day of election of such councillors, and the newly elected councillors shall come into office on the day on which their predecessors retire."
So it's 18 days, rather than 14, but without further clarification, that equates to two weeks and four days, so Monday 24 in this case.