max, that seems a bit extortionate, you are right!
A prepayment meter (one you put credit onto) shouldn't be able to debit your account for outstnding debt, as you can't use gas you don't hve-the supply is just cut off! We had this problem with electric, and they tried to come up with all sorts of excuses, but at the end of the day, you can't use what you don't have!
As long as your landlord isn't a ****, you should be able to phone round and change your supplier to one more reasonable. That, or when you do get in touch with Phoenix (I haven't heard of them, must be a Norn Iron thing!) ask if they hve a "light user" tariff or something.
It doesn't seem right that you are paying so much for so little. We pay around �35/month in winter, and half that in summer, for our gas, which fuels our hot water and our heating. Take frequent meter readings too, and stand up for yourself if they try and fob you off. I have no doubt you will! But I have had more than my fair share of hassle from utility suppliers since I first moved into my own place 5 years ago, and I hate to see people being ripped-off!
xx