Thanks Stuart.
I am unclear why 4.6.12 fixed the issue for some users, but not others if this is an iOS problem.
I understand that it could be an iOS problem; however we also have an Android user reporting what seems to be a similar issue.
IPS also said it was a problem with my server before later deciding there was a software fix / update which did solve the problem for some clients.
Unless the other clients with this problem have the same server build that I do, the only common thread here is IPS.