try using baking soda along the areas were they march.dont bother using rentokill because there only going to appear again nxt yr.if u have to use the profesional stuff get it urself itl be a lot cheaper than using re ntokil. heres a link to some profesional ant killer
http://www.domyownpestcontrol.com/ants-c-1.html
u can buy the good stuff on ebay