Thanks for your reply. I think this is what I did. I uninstalled everything on my machine, then installed "forticlient_vpn_7.0.0.0018_amd64.deb", downloaded from the website, but after the install I still get the message:
FortiClient SSLVPN is unavailable: FortiClient VPN trial has expired.
Please contact your administrator or connect to EMS for license activation.
Perhaps there has been a step I haven't followed properly on the uninstall?
Many thanks for your help.