First - thanks for the reply, it's nice of you to lend an ear
What I mean by 'single firewall IP' is that there are multiple clients on a number of private LANs, each behind their own firewall with a single public IP, who need to access the VPN server which has a public IP on a seperate subnet. The firewalls on the client networks are in this case simple NAT capable routers.
The problem is that I can only get one client per LAN/router/public IP to connect, due to the combined limitations of PPTP and NAT, after that almost all the reply packets from the server get sent to the first client machine.
On the last note, it's the only solution I've found so far too, if only the clients could run Linux too, it would be so simple!