Who decides on stop-gap head?
This appointment is entirely a matter for the governing body, although it may, of course take advice from anyone whom it wishes to consult, including the head and, where appropriate, the local education authority.
In some schools, where there is a clear order of seniority, it is the expecation that the senior deputy will automatically inherit the mantle, but, in others, especially where several deputies rotate responsibilities, the position is less clear.
There is nothing to prevent the governors allowing more than one person to take turns in the top job, thus allowing them to gain valuable experience.
The important thing is that the governors should give priority not to individual needs but to the best interests of the school and its pupils.