The rest of the staff are not all able to leave at 4 as there are clubs etc
Surely this only applies to staff who are volunteering to run a club or are paid to do so. In which case, they are choosing to be there, no?
there have been safeguarding issues with no-one senior left in charge
There doesn't need to be a DSL on site after normal school hours (senior doesn't matter: they'd need to be a DSL). In the event of a safeguarding incident, I'm sure it would be easy enough to call one of the school safeguarding leads or, if they are unavailable, contact the LADO. There are clear procedures for when safeguarding leads aren't contactable, during the day or not, and all staff should understand them.
I think maybe as the school is going downhill in many other ways and it felt that maybe this was one of the reasons.
I would say be careful with this though as if you are looking for things to support an existing viewpoint, you'll find them, objectively real or not. It's common, when a new head takes post, for parents and staff to go through a period of dislike and thinking everything is getting worse. Usually, it's just because it's a period of change and, generally, people dislike change.