Right, seem to have sorted it.
simonk83@ubnt# show vpn l2tp remote-access { authentication { local-users { username simonk83 { password **** } } mode local } client-ip-pool { start 192.168.12.191 stop 192.168.12.200 } dns-servers { server-1 8.8.8.8 server-2 8.8.4.4 } ipsec-settings { authentication { mode pre-shared-secret pre-shared-secret ****** } ike-lifetime 3600 } mtu 1024 outside-address 58.179.***.**
Not sure if it was the client IP pool that I stuffed up.
After I confirmed it working I changed the outside-address to 0.0.0.0 and that seems to be working as well, so I assume that's solved for my dynamic IP changing from time to time?