The underspend issue is a different matter; one the one hand schools are expected to fund major capital repairs but without being seen to be holding a 'surplus'. This whole 'surplus' issue arose years ago when someone asked the Govt how much money were schools stashing away without bothering to ask why..... the issue became very political, which is why there is an arbitrary 5% cap on carry-over.
If the school can demonstrate why they are building a surplus they may be allowed to keep it; at the school where I was a governor, we invested income from the sports facilities for several years running in order to accumulate enough money to pay to resurface an all-weather pitch... this was a condition placed upon us by Sports England, The FA, and a number of bodies who gave the school a grant to build the all-weather pitch in the first place.
School accounts are regularly audited, and rightly so as it is public money they are spending. A school should always be able to justify why money is spent...... but in my experience as a governor (and a school employee) I have yet to come across any audit which has questioned the staffing levels or roles & responsibilities of those employed by the school. For that matter, I don't remember Ofsted questioning this either.