By writing a script that tests the DAYOFWEEK and on Mondays 'manually' (that is, by using VB or equivalent routines in the script itself) deleting ...\path to\diff\*.*
This wouldn't necessarily need to be an installable script; you could make a stand-alone script and call it via Programs-Before (beware of calling from Programs-After if there is a chance the day-nr may have rolled over by then)
There's no feature to do so built-in, as normally people want to keep the recent diff copies in case one of them contains a version of the file earlier than the one now in \full\ and which they want to get back to. Which they normally can until the same day arrives 'next time around', at which point those backups are flushed by the appropriate setting/action in the Fast Backup settings, assuming the folder or Zip name is constant each day. Unfortunately, in the circumstances you describe, if the timing is wrong, yours won't be.
There is a setting on Copy/Delete > Advanced to delete ALL files in Destination folder (or in your case, the single-Zip) pointed to by the Variables in the Destination string). Problem is, in the circumstances you describe, if you miss the appropriate time-slot, it won't evaluate/point to the surplus/aged Zip next run anyway. Plus, that setting would also be applied on a 'full' run, although it probably won't matter if it uses a %DATE% variable (it would look straight past older 'full' backups whose name/date doesn't match; I mention in case anyone else is reading this who isn't using %DATE%...)
This wouldn't necessarily need to be an installable script; you could make a stand-alone script and call it via Programs-Before (beware of calling from Programs-After if there is a chance the day-nr may have rolled over by then)
There's no feature to do so built-in, as normally people want to keep the recent diff copies in case one of them contains a version of the file earlier than the one now in \full\ and which they want to get back to. Which they normally can until the same day arrives 'next time around', at which point those backups are flushed by the appropriate setting/action in the Fast Backup settings, assuming the folder or Zip name is constant each day. Unfortunately, in the circumstances you describe, if the timing is wrong, yours won't be.
There is a setting on Copy/Delete > Advanced to delete ALL files in Destination folder (or in your case, the single-Zip) pointed to by the Variables in the Destination string). Problem is, in the circumstances you describe, if you miss the appropriate time-slot, it won't evaluate/point to the surplus/aged Zip next run anyway. Plus, that setting would also be applied on a 'full' run, although it probably won't matter if it uses a %DATE% variable (it would look straight past older 'full' backups whose name/date doesn't match; I mention in case anyone else is reading this who isn't using %DATE%...)