Pointers / info in setting up a backup sequence that will overwrite after n runs

Anyone point me in the right direction for this?

I (just) upgraded to BESR 2013, and I would like to configure backups so that they overwrite after x number of backups, rather than filling up the external drive. Leaving me to note it's full and then manually delete. (not efficient)

This is configured on two different machines, one cycles fine, the other fills up every few months. Try as I might, I cannot see  what the difference is between the settings on the machines.

 I am sure this is staring me in the face, but as is usual with these things, I need the obvious pointing to me.

Thanks in advance.

P.