potterton puma 100e gas bills?

i have recently had a gas bill on a potterton puma 100e which said that i had used 1080 units of gas in 63 days approx 18 units a day and for £380,however for a 23 day period before i used approx 2.5 units a day, i have checked the meter on a daily basis since and find that i use 2.5 units a day,during the 63 dy period i used the heating/water etc as normal,does anyone know of a problem which uses excessive gas and then seems to right itself any help would be appreciated