Hey,
Got a work around in the end. I turned the firewall and everything back to normal so the captive portal worked. I then setup a Virtual IP for my whole WAN range and then added a NAT 1:1 rule for each IP address. Since this cant include the WAN IP address I couldnt just do a /24 range. I therefore setup NAT 1:1 for a /25, /26, /27, /28, /29 and a /30 range so that I covered 192.168.0.4-255. Now if a PC on my LAN network with a IP address of 192.168.1.123 uses the internet the traffic appears to come from 192.168.0.123.
There was probably a better way to do this but it worked for me.