Make sure each teacher is in the same group as their students. Make sure your course settings are set to groups - separate or visible - as long as groups - and make sure the assignments are also set to groups. That way when the teachers go to grade they will see a dropdown menu and they can filter for their own group.
If you want to take it further and only let them see members of their own group then follow the instructions in Groups FAQ (link) - the question:How do I restrict a teacher to view only information about the groups that they are in?
I had got most of that ... I will go an track down one of the teachers and check it out.
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