Quote Originally Posted by bazza

Now I've adjusted my awr retention policy and its taken affect (off topic but how often is this done and does MMom do it?).
By default, MMON process takes snaps every one hour and retains for 7 days. You can see your current settings with,

select snap_interval, retention
from dba_hist_wr_control;

About snapshots not getting cleared, check if you have baseline defined on them. MMON does not purge baseline snapshots even after retention period. You then have to purge such snapshots using DBMS_WORKLOAD_REPOSITORY package.