I still could not get it to work ... not sure why ....
However, I have a work around for the moment ... Duplicate the assessment task and restrict access to the task according to the groupings. That seems to be what is I need ... unless I have to change the task itself for some reason!
It does seem a little arcane ... I can import/associate students with cohorts, I can enter students on a course by cohort .. but then I have to create several groups for each course, based on these same cohorts, rather than simply filtering by the cohorts directly in the mark book ... still confused !!!
Still ... thanks for the support ... I will persevere