Can you help with Problems with gas meter?

I recently moved in with my friend.We're on a gas meter due to a debt from her previous tenant.Problem is when we put money in it takes 55% for the bill.Surely they can't do this.We're always in high rate emergency because of this.Also might be something else wrong.We've put 50 quid in this week alone.Last night I put 20 in.It gave me 3.70 credit after it took for debt and emergency.I had a bath and radiator on for a few hours.Checked and there was loads left.I turned boiler off and radiators and went to bed.Got up its all gone.We have a combi but how can it all go if i'm not using it.Is it a leak or is someone tapping it.Is that normal.Please help.