I would contact your supplier again and ask excactly why there is a debt on your meter. If need be ask/demand this in writing so you cna properly digest the infomation. There are circumstances where debt can accrue on a prepayment meter.
Assuming for now that the debt is valid i would also enquire into the debt repayment value? it may be set quite low which would explain why the debt balance is lingering, look into the possibility of increasing this to clear the debit balance early.
If your looking to change supplier you will need to clear any monies that are owed before you change supplier, your current supplier would be fully entitled to object to another supplier taking supplier if there is a debt on your account longer than 90 days.
Now "entitled to" doesn't mean they will, and it doesn't mean they have to. There is a real possibility that they may allow the change of supply. This may sound great at first, however your supplier will then request full payment of the outstanding balance, and would not normally give more than 3 months for this repayment. In which case they tend to pass the debt to a debt collection agency.
Now all of this maybe completely irrelevant if they cannot justify the debt. Perhaps collect this infomation and post back and we cn take a look at the situation again given more infomation.
Asfor ebiico, i would check out the reviews section of each supplier, they seem tobe getting really good feedback so they must be doing something right. They may or may not be the chepest supplier for you, i would follow Shiverkitten's advice find out your annual consumption (your supplier will have this infomation) and enter this into a comparrison site's comparrison tool.
The answers I give you may not want to hear. The answers I give may not be easy. The answers I give will be the truth. If your prepared for this - ask away!