Sorry for not ploughing through the forums a bit more,
this was anwsered by Wiregr in another thread,
iptables -t nat -A POSTROUTING -s 172.16.42.0/24 -o usb0 -j MASQUERADE
iptables -A FORWARD -s 172.16.42.0/24 -o usb0 -j ACCEPT
iptables -A FORWARD -d 172.16.42.0/24 -m state --state ESTABLISHED,RELATED -i usb0 -j ACCEPT
Worked a charm,
Bag.