I found the solution to my problem.
The parameter NTAP_SNAPSHOT_DISABLE has to be 'N' for Snapcreator to clean up snapshots. If this parameter is enabled, Snapcreator will just skip the snapshot delete process
I was in the impression that this parameter needed to be 'Y' because I was taking snapshots externally through Snapdrive commands, so I didn't want a normal ZAPI snapshot create also. This was a difficult one to find because snapshot creation was working and only the snapshot deletion was being skipped (not honouring my policy).
Grtz,
Tom