I have not previously heard of a committee solely concerned with appointments, though of course most schools will have a staffing committee that reviews staffing needs, decides or makes proposals on the staffing structure and deals with any more specific problems which might arise with staff. I presume you have one of these as well. A governing body is entitled to set up any committees it thinks necessary, but I don't think there is much of a case for having one on the appointment process.
First, in the case of the senior vacancies, you say the committee sits only sporadically, but the governing body as a whole should meet for such an important task, and be specifically empowering a fresh selection panel as these occasions arise. It is certainly not appropriate to exclude staff governors as a group from senior appointments panels, and it is certainly not desirable that the same people should be repeatedly charged with this task.
I must add something about governors' involvement in the appointment of teaching staff. As you indicated, there has been a change in government guidance about this - to the effect that they expect appointments below the level of deputy to be delegated to the headteacher with effect from the beginning of this school year, though with extension until the end of April in special circumstances. Interestingly, though, the ruling is not part of the regulations but only included in the guidance. In my book that means it is not statutory.
I have heard that some schools have settled for this interpretation and therefore maintain their former practice. I would be very interested to know how widespread this is.
Whatever the rights and wrongs of this issue, if any school is still involving governors, I would not consider it wise to be so formal in identifying those who can manage a daytime interview. Many good habits would be brought to an end if this became a universal assumption.
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