Wireless Range Extenders
What you need there is a wireless range extender. There are several on the market, but it's probably best if you could find one made by the same people who made your router (they usually play better together, in my experience).
A single extender shouldn't cost more than £30. Some companies even offer small, wall-mounted extenders (about the size of the average plug), if you don't fancy having an extra normal sized router lying around the place.