Resolved
For anyone who has a similar issue: I tested with a test ibay set with the same 'everyone' permissions and copied a few files into it. Changing the ibay group settings does not affect the files in the ibay at f/s level.
It does however change the access from outside the ibay as it of course should. And it's almost instant.
Reverting the group permissions to group/group r/w for the main group consisting of all employees who should have regular access worked as intended.
So if anyone else inadvertently changes ibay access permission to something too loose then reverting it should not be an issue.
No-one will admit having changed the permissions, on this and another server, while most are set up properly, but that's another sort of problem!
Thanks.