I have for some time been unable to make changes to /etc/hosts stick; each time I reboot, the file is reverting to its previous state.
It appears that Cisco AnyConnect is at fault, it keeps private copy of the hosts file at /etc/hosts.ac and overwrites /etc/hosts each time it starts (which in my case is simply each time I reboot). The solution is simply to make any changes to both files at the same time.
(thanks)