How do I use RDP through ISA 4 VPN over L2TP/IPSec

I’ve setup a VPN server using MS ISA 4.
I’ve configured both PPTP & L2TP/IPSec connections.

I can remote desktop to any machine on the internal network when connected using PPTP but when I connect using L2TP/IPSec I can’t get a connection to any machine. Everything else works ok on both protocols, file access, outlook etc, but remote desktop doesn’t work on L2Tp/IPSec.

The server OS is Windows 2003 with all updates installed.
ISA server 4 also has all updates applied.

Any idea’s?