I think I have found the issue
This allows a user to view a feedback activity
This capability is allowed for the default roles of manager. teacher, non-editing teacher, student and guest
In Moodle 2.4.6 onwards, feedback links on the course page and in the navigation are only displayed for users who have this capability. (Previously links were displayed for all users, and users without the capability obtained an error message when they clicked on a link.)
Moving 2.4 to 2.5 I have to enable View a feedbackmod/feedback:view for students, whereas before I didn't have to.
I hope this helps someone else And apols to those for whom it was wheel-spin