Expensive gas bill

Only been in property 6 months, charge over 6 months is £648, electricity seems normal, bill for first 3 months for gas (though estimated) was £51, next bill £435 for the next 3 month period. Seems a tad expensive considering its a 2 bed property that I'm in alone and I'm at work 5 days out of 7 and generally out the rest of the time. It does however work out at £108 a month, so it may be right, just seems excessively high - the house does have problems, constantly cold all the time despite the heaters being on. It's with SSE, wonder if any others have had experience/or what are the size of your bills?