<human>: Hey, I'm currently having some issues with my landlord, I'll run through them.

I was in a other one of his properties last year, and as part of the tenancy agreement I paid £346.66 a month of rent and bills money to him. 
It turns out that apparently my housemate made a mistake and did not change gas provider when he changed energy provider, and so we are being told we owe the gas company in excess of £500. 

My issue is the fact that I was under the assumption that according to the tenancy agreement, I was paying my share of any bills, and so if we literally were not paying for gas, then the gas money should still exist in my landlord's finances? 

What legal ground do I stand on here Reddit? 

Oh, and I literally have no money to pay this deficit if I wanted to, and I definitely don't want to pay for something I was told I had already paid for.

Could you distill the essence of the text into 1-3 sentences?
<bot>: landlord says we owe the gas company money for all the gas last year, but took money from us all year for the gas which turns out not to be paid for - help