I've solved the crisis hopefully, brought another WRT54GL and I'll DDWRT my current one, this means I can then operate a client-bridge (as DDWRT wiki has quite a bit of info on setting this up and someone has the same setup as me)
Standards, we really need them!
You do need a router, because I need to connect the 2 LANs wirelessly, so the router on the new temp lan needs to acts as a client-bridge which in effects make it look like it's wired, allowing the main router to offer DHCP to the new lan and the DDWRT router can handle all the modifying of packets to offer transparency.
Here is a good diagram of what the network would look like.
http://www.dd-wrt.com/wiki/index.php...idge_large.jpg
If anyone else stumbles across this in future,
http://www.dd-wrt.com/wiki/index.php/Wireless_Bridge
Is a good place to start to understand this