If I use
SELECT
COMPARE_REPORT_MASTER.RESULT
FROM
COMPARE_REPORT_MASTER(:URL1, :SITENUM, :SITETYPE, :SESSION_ID)
to test my stored procedure it works fine.

If I try to replace any of the paremters with set variables it seems
to fail, i assume I have the wrong syntax.

if all the parameters are strings would it be
SELECT
COMPARE_REPORT_MASTER.RESULT
FROM
COMPARE_REPORT_MASTER('URL1', 'SITENUM', 'SITETYPE', 'SESSION_ID')

as this doesnt seem to work