Firewall/NAT

I am trying to setup l2tp/ipsec connectivity to m0n0wall (1.3b15) by having m0n0wall handle ipsec and a windows 2003 server the l2tp part.

This used to be working with my previous firewall (OpenWRT). All that was required was to disable ipsec on the windows server such that it would accept plain l2tp requests and on the firewall side a road warrior setup for ipsec and DNAT which would forward l2tp requests from the ipsec tunnel to the windows server.

On m0n0wall I have the ipsec part working (added logging firewall rule to verify that l2tp packets are received from tunnel) but cannot work out how to setup port forwarding.

I have tried inbound nat with interface set to WAN but to no avail (no traffic arrives on the windows server). It appears to me that inbound nat should have an option to choose the ipsec pseudo-interface.

Any ideas on how to apply inbound nat to packets arriving from an ipsec tunnel?