The monitoring officer isn't involved in elections; merely in councillor/council behaviour. When a casual vacancy occurs (i.e. a councillor resigns) It is the role of the elections office at your principal authority to issue the relevant notice of vacancy which invites members of the public to request an election. If no such request is received within the statutory period, then the council is free to co-opt. If, however, this is a vacancy which has resulted from insufficient candidates at the elections May, the council can co-opt without the requirement for a notice inviting members of the public to request an election (subject to the council being quorate after the May elections) so the council can co-opt within 35 days of the actual full election. The elections office should be giving advice on the next steps.