My central heating, gas for my house.?

I have a Ravenheat CSI 85 AAAT Combi-boiler and im not shore if its using too much gas or wether my supplyer is charging me too much, im on a pay as you go gas meter as i was renting and have now bought the house, im putting on the card at least �20 a week i dont no wether when i put the boiler on its using stupid amounts of gas or what..... in the summer when i dont have the heating on �20 lasts for ages mabey a month as i only use the cooker and hot water please help somone its draining my wallet lol cheers.