I've just realised where I've been going wrong! (duh)
Under Moodle Modules > Activties > Manage activties it is necessary to have the eye CLOSED on the Mahara assignment mnet module.
Then when you go to Add an activity to a course, under Assignments you see Mahara Portfolio as the bottom option.
I had been missing this as with the eye OPEN on the Manage Activities > Mahara assignment mnet module my attention on the Add an activity drop down list had immediately been drawn to the unwanted Mahara assignment mnet module option and I had missed seeing the Mahara portfolio option altogether.
Thanks very much to Dajan for inadvertently helping me realise my mistake.
Moodle networking (MNet)
Cannot get submit Mahara view to Moodle assignment working
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