My school already offers a "period 7"
lesson which runs from 3.25 - 4pm.
This is used for study support, after school clubs (different each day), tailored learning (ie specific year 11 sessions aligned to subjects) and, for a very small number, an actual lesson for those students who are taking an additional gcse that can't be fitted onto the timetable.
It works well because it's optional and uptake is high because it's a varied menu.
It's also written into the school expectations that if a student is not reaching their potential, then those students will be required to attend one or more of those period 7 lessons that have been allocated until they have caught up. This is in the home learning contract with parents.
I think this works very well because it allows extra intervention, enrichment and catch up as well as a back up. It also means that teachers can opt in when it's suitable (many teachers will have meetings, detention duty, training at this time so can't do this every day)
Most teachers in my school don't leave until around 5ish anyway, so this flexible offer forms part of their contracted hours.
However if you start to make it compulsory, then it loses its effectiveness for students (they would miss out on catch up time to do an arbitrary timetabled lesson?), you would lose enrichment and after school provision and teachers would be required still to do meetings, detentions and training but later in the day.
A blended offer could be really suitable (and is, in my school) but I would object to another lesson a day.