Has anybody successfully used the Andrews & Arnold L2TP service on a Draytek router before? Details here:  https://www.aa.net.uk/broadband/l2tp-service/ .

Specifically, I am using a Vigor 2865.

The idea being to gain a static, public IPv4 (Or v6, actually) whilst using another generic underlying internet connection. I have noticed the L2TP option in the WAN settings on the Draytek, however I think this is for connecting to a primary ISP. Obviously in this case the usual method should be used to connect to the ISP, whether it be PPPoE, DHCP or anything else, and then the L2TP tunnel to AAISP should run OVER this connection. 

Thus, I am thinking the Lan-2-Lan VPN feature could be used for this in L2TP mode. I am sure this would work in NAT mode for outgoing traffic, however what happens if I want to allow incoming traffic destined for an AAISP static IP in to a server connected to the Draytek's LAN? How can this be achieved?

I don't currently have an AAISP L2TP account to experiment with, just hoping for some ideas as I may pursue this in the future.