Looking at the plan, 73% of the estimated cost comes from 2 joins and a 'not null' filter, which I've reconstructed as

Select

AUND.AUND_INBOX,

RPT_SQL_CL.dbo.FFG_FX_JUST_DATE(A.WF_START) as Expr1007

from

RPT_SQL_CL.dbo.IREP_T_WF_INFO as A

right join -- 13,988,837 rows (19% cost)

RPT_SQL_CL.dbo.IREP_T_UNDW_STAFF

on

A.ASSIGNED_TO = RPT_SQL_CL.dbo.IREP_T_UNDW_STAFF.UNDS_STAFF_INBOX

join -- 13,980,811 rows (50% cost)

RPT_SQL_CL.dbo.IREP_T_AGENCY_UNDW as AUND

on

AUND.AUND_AGENCY_CODE=A.AGENCY_CODE

where

A.ASSIGNED_TO IS NOT NULL -- 22,079,800 rows(14% cost)

The hash join (for the DISTINCT) adds another 14% to the cost.

To do much further analysis really needs the original source of the view IREP_V_DAILY_COUNTS_TERRRITORY (reconstructing it from the plan takes too long!) and DDL for the tables IREP_T_WF_INFO, IREP_T_UNDW_STAFF, IREP_T_AGENCY_UNDW, IREP_T_EMPLOYEES and IREP_T_CDIR_EMPLOYEES.

Also I'd be interested to see the source of the User-defined function FFG_FX_JUST_DATE since, although it doesn't add the the estimated cost, it appears the estimate is for it to be called over 13 million times.