This happens quite often.
I would contact customer services and explain the situation as I wouldn't be surprised if the Harlow office is a franchise and don't really care what your problem was when you weren't able to drop keys off to someone.
I'm surprised also that the reservation didn't spot that the branch was closed upon drop off time. It usual say well time of closure.
I think an extra day is not unreseanable, 2 seems harsh!