I agree you should try to get this pre-payment meter removed, as they are the most expensive way of paying for gas or electricity.
However, it is possible that the meter has been set to recover a debt from the previous owner/tenant, in which case it will be taking more than the actual gas cost. Also, there is a standing charge on these meters, so they do rack up a cost if you are not using the gas (I think this is a relatively small part of the bill, but the supplier should be able to tell you exactly what the tariff is).
If they refuse to change the meter or do anything about it, go through their complaints procedure and then to the Energy Supply Ombudsman (see website) if necessary. A phone call to the latter may help anyway - they can probably give you info. on how to go about getting the meter changed.