Firewall/NAT

I have recently purchased a M0n0wall firewall running on a LinITX Alix 2D1 LX700 (3NIC) and am having difficulty configuring remote VPN access using a standard XP VYN client.

If I first create a dial up connection to the internet via PSTN and then make a VPN connection to my firewall it works fine. However, if I connect to the internet via broadband, ADSL or megastream and then attempt to make the VPN connection it is rejected (VPN Error 619)

Virgin Media (my boadband supplier) have assured me their Cable Modem (Motorola Surfboard 4200) does not block any inbound protocols.

My m0n0wall device is connected directly to the cable modem via an ethernet cable and I spoof the MAC address held in their database for authentication.

I have a PPTP VPN rule configured to allow any protocol from PPTP Clients  from any source port to any destination using any port.

Any help you can give me would be much appreciated.

Because it works using the PSTN connection, I would say that the Mono config is correct.  Have a look at firewall settings on the client machine.  If they are interface specific then it could be in there.