I may be wrong, but if you just connect the WAN port to one of the LAN ports on the Plusnet router, make sure the two routers have different IP addresses and disable DHCP on one of them, don't you get what you want?
Edit: Actually I'm not sure about the WAN port bit. It will definitely work if you connect one of the LAN ports on your router to the Plusnet router, because that's what I have done many times in order to have multiple wireless access points using spare routers. You can use the remaining LAN ports and wireless access on either router to connect.
Edited by kasg (Mon 07-Oct-13 20:29:16)