Easiest way mate is leave the modem where it is and get a longer network lead.
http://www.maplin.co.uk/Search.aspx?criteria=utp&source=15&SD=Y
alternatively u could extend the cable feed with one of these.
http://www.maplin.co.uk/Module.aspx?ModuleNo=29537&criteria=f%20plugs&doy=12m5