Automated backup not overwriting

I have our automated backups set to keep five and then over write. It isn't happening. It does the backups but it is not over writing so we have more than five of each course and have to remember to delete manually.

Is it a fault do you think? I saw another thread where someone was asking how to bulk delete automated backups implying their's wasn't over writing either.

I'm on Moodle 2.4.3 on a WIndows 2008 R2 server with MySQL. Is it a bug or have I got something wrong with mine.

Thanks

Heather