See further below for GUI / Ubuntu / Network Manager instructions

Linux / xl2tpd

Ensure the following kernel options are set or the corresponding modules are available:

Install xl2tpd and pppd on your Linux router.
Edit /etc/xl2tpd/xl2tpd.conf to contain the following:
[lac aaisp] lns = l2tp.aaisp.net.uk require authentication = no pppoptfile = /etc/ppp/options.aaisp
Create /etc/ppp/options.aaisp containing the following (obviously change the name and password to match your L2TP login details):
+ipv6 ipv6cp-use-ipaddr name xyz@a.X password Your_xyz@A.X_password noauth
Create the xl2tpd control file:
mkdir -p /var/run/xl2tpd touch /var/run/xl2tpd/l2tp-control
Start the xl2tpd service (for systemd, use service command for older RC systems):
systemctl start xl2tpd
Tell the daemon to connect to aaisp:
echo "c aaisp" > /var/run/xl2tpd/l2tp-control

This should give you a new PPP device which encapsulates the L2TP connection.

Troubleshooting

Logs are written /var/log/daemon.log If you see "No such tunnel", you might want to try restarting the daemon

First, install the packages for the l2tp client and the network manager controls:

apt install xl2tpd network-manager-l2tp network-manager-l2tp-gnome

If the packages are not found, you may need to add some PPA's, eg one or both of these may be required:

sudo add-apt-repository ppa:nm-l2tp/network-manager-l2tp
sudo add-apt-repository universe
sudo apt-get update

Then, using Network Manager, create the L2TP connection:

Add a L2TP connection: (pic below)
1. Click the Network icon on the task bar
2. Click Settings
3. Next to 'VPN', click the + icon
Enter in the settings: (pic below)
1. Name: anything, eg AAISP
2. Type: Password
3. User name: your login we gave you, eg xxx@a.1
4. Password: the password we gave you
5. Click Apply
Connect/disconnect via the network icon on the taskbar (pic below)