I'm not sure about hooking the router up to the modem through homeplug. I think you need to plug the router straight into the modem.
One option I can think of is to leave the router where it is, and turn the wireless option off. Next get a 'wireless extender' set.
http://www.ebuyer.com/product/106821 (these are 54 mbps, I'm sure there's faster ones available).
You connect one of them to the router, and the other one will provide you with a wireless network at the back of the house (simply plug it into the mains at a convenient location. Alternatively, just get a couple of homeplugs, one for every device you want to connect.
Edit: Beaten