Mick,
Tech support has come back with the following:
The “remember me” cookie expires after 2 weeks, so every fortnight they will be asked to log in again. Also if they log in using another browser or another computer that will break the “link” and they’ll need to log in again.
I presume that the cookie renews itself every time you log in as i don’t have this problem.
Hope this is of some help.
Colin