Theres not much you can do with the router they have supplied, its only limited to whats its been designed to. What you can do is buy another router which will pick up the signal, then send it on again essentially extending the signal
Any decent electronics shop can sell you a wi-fi repeater which forwards a signal via the mains circuit in your building.
Alternatively, BT are currently offering their own version at half price:
http://www.shop.bt.com/products/bt-dual-band-wi-fi-extender-600-8W46.html
(I've come across that one in use at several B&Bs where I've stayed and it seems to do a good job. You don't need to be a BT ciustomer to buy it or to use it - although you already are because PlusNet is a BT service anyway!)