For clarity, this document outlines on how an Extraordinary PC meeting is called: Standing Orders Chapter 6. or in this document: GOV.UK Chapter 8/9.
Below is an extract from the PC Standing Orders.
6. EXTRAORDINARY MEETINGS OF THE COUNCIL, COMMITTEES AND SUB-COMMITTEES
a The Chair of the Council may convene an extraordinary meeting of the Council at any time.
b If the Chair of the Council does not call an extraordinary meeting of the Council within seven days of having been requested in writing to do so by two councillors, any two councillors may convene an extraordinary meeting of the Council. The public notice giving the time, place and agenda for such a meeting shall be signed by the two councillors.
c The chair of a committee [or a sub-committee] may convene an extraordinary meeting of the committee [or the sub-committee] at any time.
d If the chair of a committee [or a sub-committee] does not call an extraordinary meeting within (3) days of having been requested to do so by (2) members of the committee [or the sub-committee], any (2) members of the committee [or the sub-committee] may convene an extraordinary meeting of the committee [or a sub-committee].