Further to my initial post about just restarting the VPN module, which does the job, albeit a bit of a hammer to crack a nut..
Here are a few more usefull commands.
To list all your current VPN Tunnels – IKE / IPSec etc..
Open a command line on the firewall, go into expert mode and type: vpn tu
You then get the option to list tunnels, or kill them by peer ip or user (client) or just all.
This gives you a much more granular method to debug and view / kill tunnels on a per peer type basis.
To switch VPN debugging on, which logs to $FWDIR/log/vpnd.elg use the following:
vpn debug on
vpn debug ikeon
vpn debug trunc (this command is the equivalent of the above 2)
To switch debugging off use:
vpn debug off
vpn debug ikeoff
You can then export the log file to another machine for analysis.