n3mo: not sure what you are worried
What suppliers do is "load" the meter to collect a normal standing charge each month plus a monthly payment to be set against a "debt". A positive balance showing on the meter will indicate that you can use power even when the debt hasn't been repaid in full. I'm not sure if some meters will also show debt details or the total monthly/weekly/daily standing charge, but it's worth pushing buttons to see what it will show. It should be possible though to see how many "units" (or possibly cubic meters with gas) are used in a week or a month and then calculate how much they should have cost and then work out the difference with what was paid.
Suppliers have a responsibility to send out regular statements that should have all the relevant details on but suppliers don't always honour their responsibilities, particularly when they make mistakes.
However, if all else fails, starting the procedure to switch supplier should either make the supplier declare the account situation (or their version of it) or possibly, if the current supplier doesn't claim that there is a debt, it will give Trfc06 the chance to start with a zero balance with a new supplier. If the current supplier declares that there is a debt to pay, holding a copy of a letter as mentioned in my last post will help the situation.