routing all traffic for port 80 via vpn connection? (newbie problem..)

I have a vpn connection set up using the linux pptp client software.
At present, the routing table is set up to route all traffic through
the pptp interface (ppp1). I have been struggling hard with iptables
to try to route only outgoing web traffic through the interface.

I have installed the ROUTE patch for iptables, and tried the
following:

iptables -A POSTROUTING -t mangle -p tcp --dport 80 -j ROUTE --oif
ppp1 --continue

but this just disables access to the web completely.

I am sure I am doing something very stupid, but I don't entirely
understand gateways, interfaces etc. and iptables is new to me (my old
setup used just the route command).

Any help or advice would be gratefully received.

James